The invention claimed is:1. A method, comprising:receiving, via an end-to-end encrypted session between a sending device and a receiving device, a message including a reference to an image;establishing a connection to a blob store, the blob store holding an encrypted thumbnail of the image and additional encrypted data that, when combined with the encrypted thumbnail, results in a full-quality version of the image;retrieving the encrypted thumbnail from the blob store, wherein retrieving the encrypted thumbnail comprises:downloading a data chunk including at least some of the encrypted thumbnail; receiving a keyed-hash message authentication code (HMAC) in connection with the data chunk;calculating an HMAC over the downloaded data chunk;comparing the calculated HMAC to the received HMAC to verify that the encrypted thumbnail has not been altered;decrypting the thumbnail; anddisplaying the thumbnail;receiving a request to display the full-quality version of the image;retrieving the additional encrypted data from the blob store, decrypting the additional data, and combining the decrypted additional data with the thumbnail; anddisplaying the full-quality version of the image.2. The method of claim 1, wherein retrieving the encrypted thumbnail from the blob store is performed automatically in direct response to receiving the message.3. The method of claim 1, wherein the encrypted thumbnail and the additional encrypted data are stored together in an image structure, and the message including the reference to the image comprises a location in the image structure that divides the encrypted thumbnail from the additional encrypted data.4. The method of claim 3, further comprising:receiving a request to forward the image to a second recipient; andretransmitting the location in the image structure that divides the encrypted thumbnail from the additional encrypted data while refraining from retranscoding the image.5. The method of claim 1, wherein retrieving the encrypted thumbnail from the blob store and retrieving the additional encrypted data from the blob store are performed together in response to a single request for the image referenced in the message.6. The method of claim 1, wherein the image is a progressive JPEG.7. A non-transitory computer readable medium storing instructions configured to cause a processor to:receive, via an end-to-end encrypted session between a sending device and a receiving device, a message including a reference to an image;establish a connection to a blob store, the blob store holding an encrypted thumbnail of the image and additional encrypted data that, when combined with the encrypted thumbnail, results in a full-quality version of the image;retrieve the encrypted thumbnail from the blob store, wherein retrieving the encrypted thumbnail comprises:downloading a data chunk including at least some of the encrypted thumbnail; receiving a keyed-hash message authentication code (HMAC) in connection with the data chunk;calculating an HMAC over the downloaded data chunk;comparing the calculated HMAC to the received HMAC to verify that the encrypted thumbnail has not been altered;decrypt the thumbnail; anddisplay the thumbnail;receive a request to display the full-quality version of the image;retrieve the additional encrypted data from the blob store, decrypt the additional data, and combine the decrypted additional data with the thumbnail; anddisplay the full-quality version of the image.8. The medium of claim 7, wherein retrieving the encrypted thumbnail from the blob store is performed automatically in direct response to receiving the message.9. The medium of claim 7, wherein the encrypted thumbnail and the additional encrypted data are stored together in an image structure, and the message including the reference to the image comprises a location in the image structure that divides the encrypted thumbnail from the additional encrypted data.10. The medium of claim 9, further storing instructions for:receiving a request to forward the image to a second recipient; andretransmitting the location in the image structure that divides the encrypted thumbnail from the additional encrypted data while refraining from retranscoding the image.11. The medium of claim 7, wherein retrieving the encrypted thumbnail from the blob store and retrieving the additional encrypted data from the blob store are performed together in response to a single request for the image referenced in the message.12. The medium of claim 7, wherein the image is a progressive JPEG.13. An apparatus comprising:a network interface configured to:receive, via an end-to-end encrypted session between a sending device and a receiving device, a message including a reference to an image; andestablish a connection to a blob store, the blob store holding an encrypted thumbnail of the image and additional encrypted data that, when combined with the encrypted thumbnail, results in a full-quality version of the image; a processor configured to:retrieve the encrypted thumbnail from the blob store, wherein retrieving the encrypted thumbnail comprises:downloading a data chunk including at least some of the encrypted thumbnail;receiving a keyed-hash message authentication code (HMAC) in connection with the data chunk;calculating an HMAC over the downloaded data chunk; andcomparing the calculated HMAC to the received HMAC to verify that the encrypted thumbnail has not been altered;decrypt the thumbnail; anddisplay the thumbnail;receive a request to display the full-quality version of the image; andretrieve the additional encrypted data from the blob store, decrypt the additional data, and combine the decrypted additional data with the thumbnail; anda display configured to display the full-quality version of the image.14. The apparatus of claim 13, wherein retrieving the encrypted thumbnail from the blob store is performed automatically in direct response to receiving the message.15. The apparatus of claim 13, wherein the encrypted thumbnail and the additional encrypted data are stored together in an image structure, and the message including the reference to the image comprises a location in the image structure that divides the encrypted thumbnail from the additional encrypted data.16. The apparatus of claim 15, wherein the network interface is further configured to:receive a request to forward the image to a second recipient; andretransmit the location in the image structure that divides the encrypted thumbnail from the additional encrypted data while refraining from retranscoding the image.17. The apparatus of claim 13, wherein retrieving the encrypted thumbnail from the blob store and retrieving the additional encrypted data from the blob store are performed together in response to a single request for the image referenced in the message.