What is claimed is:1. An image display apparatus comprising:a display;a processor;a memory; andat least one program stored in the memory and executed by the processor,wherein the at least one program comprises instructions which when executed by the processor cause the image display apparatus to:obtain a general image by decoding a first video content,store the general image in a first buffer, the first buffer being for storing a video image,obtain a planar-format image by decoding a second video content, wherein the general image and the planar-format image are represented by a first color space,perform graphic processing on the planar-format image to obtain a sphere-format image corresponding to the planar-format image, the sphere-format image represented by a second color space,select a partial sphere-format image among the sphere-format image based on a user input on the sphere-format image,perform graphic processing on the partial sphere-format image to obtain a partial planar-format image,change a color space of the partial planar-format image from the second color space to the first color space, for performing image quality processing on the partial planar-format image,store the partial planar-format image of which color space is changed from the second color space to the first color space in the first buffer,generate an on screen display (OSD) image,store the OSD image in a second buffer, the second buffer being for storing a graphic image,perform the image quality processing on the partial planar-format image of which the color space is changed from the second color space to the first color space and which is stored in the first buffer, andcontrol the display to display the image-quality-processed partial planar-format image with the OSD image,wherein the processor is configured to store the general image in the first buffer without performing of the graphic processing and the change of the color space on the general image, andwherein the first buffer storing the partial planar-format image and the general image is configured separately from the second buffer storing the OSD image.2. The image display apparatus of claim 1, wherein the first color space is a YUV color space and the second color space is an RGB color space.3. The image display apparatus of claim 1, further comprising a color space converter (CSC), implemented by the processor, configured to change the color space of the partial planar-format image from an RGB color space to a YUV color space.4. The image display apparatus of claim 1, further comprising an image quality processor, connected to the first buffer, configured to perform the image quality processing on the planar-format partial image received from the first buffer and output the image-quality-processed planar-format partial image.5. The image display apparatus of claim 1, wherein the image quality processing comprises at least one of contrast ratio and color improvement, gamma correction, sharpness enhancement, and jagging phenomenon alleviation processing.6. The image display apparatus of claim 1, further comprising a video decoder configured to decode the second video content to obtain the planar-format image.7. The image display apparatus of claim 1,wherein the processor is further configured to:map the planar-format image to the sphere-format image.8. The image display apparatus of claim 7,wherein the processor is further configured to receive the user input for selecting the partial sphere-format image from the sphere-format image and extract the partial sphere-format image from the sphere-format image based on the user input.9. The image display apparatus of claim 1,wherein the image quality processing on the partial planar-format image optimizes a cubic effect and a color sense of the partial planar-format image and enhances sharpness of the partial planar-format image by performing sharpness processing on edge portions of the partial planar-format.10. The image display apparatus of claim 1, wherein the processor is further configured to perform graphic processing on the partial sphere-format image by correcting distortion of the partial sphere-format image.11. The image display apparatus of claim 1, wherein the second video content corresponds to 360-degree video.12. A method of displaying an image, the method comprising:obtaining a general image by decoding a first video content;storing the general image in a first buffer, the first buffer being for storing a video image;obtaining a planar-format image by decoding a second video content, wherein the general image and the planar-format image are represented by a first color space;performing, by a processor, graphic processing on the planar-format image to obtain a sphere-format image corresponding to the planar-format image, the sphere-format image represented by a second color space;selecting a partial sphere-format image among the sphere-format image based on a user input on the sphere-format image;performing graphic processing on the partial sphere-format image to obtain a partial planar-format image;changing a color space of the partial planar-format image from the second color space to the first color space, for performing image quality processing on the partial planar-format image;storing the partial planar-format image of which color space is changed from the second color space to the first color space in the first buffer,generating an on screen display (OSD) image;storing the generated OSD image in a second buffer, the second buffer being for storing a graphic image;performing the image quality processing on the partial planar-format image of which the color space is changed from the second color space to the first color space and which is stored in the first buffer; anddisplaying the image-quality-processed partial planar-format image and the OSD image on a display,wherein the general image is stored in the first buffer without the graphic processing and the change of the color space being performed on the general image, andwherein the first buffer storing the partial planar-format image and the general image is configured separately from the second buffer storing the OSD image.13. The method of claim 12, wherein the first color space is a YUV color space and the second color space is an RGB color space.14. The method of claim 12, wherein the performing the image quality processing on the partial planar-format image comprises performing at least one of contrast ratio and color improvement, gamma correction, sharpness enhancement, and jagging phenomenon alleviation processing.15. The method of claim 12, wherein the performing graphics processing on the planar-format image comprises:mapping the planar-format image to the sphere-format image.16. The method of claim 15, further comprising:receiving an input for selecting the partial image of the sphere-format image, andwherein the obtaining the partial sphere-format image comprises extracting the partial sphere-format image from the sphere-format image based on the user input.17. The method of claim 12,wherein the image quality processing is not performed on the OSD image.18. The method of claim 12, wherein the image quality processing on the partial planar-format image optimizes a cubic effect and a color sense of the partial planar-format image and enhances sharpness of the partial planar-format image by performing sharpness processing on edge portions of the partial planar-format image.19. The method of claim 12, wherein the performing graphic processing on the partial sphere-format image comprises correcting distortion of the partial sphere-format image.20. The method of claim 12, wherein the second video content corresponds to 360-degree video.21. A non-transitory computer-readable recording medium having recorded thereon program instructions, which when executed by a computer, perform the method of claim 12.