Thought One: Your biggest barrier to adoption will be speed as you scale. If your API returns the modified image too slow, developers will quickly abandon you. So you'll need to invest heavily in RAM, processing power, and code/db optimization.
Thought Two: I suggest, if you have the storage to support it, that you take a checksum of all original images and cache the original and modified versions of them. As you scale you will find lots of people submitting the same images (celebrity pics, scenery, etc) and you could save alot of time and bandwidth by doing a checksum client-side, AJAXing it back to your server and, if there is a match, just return your cached version (if the client requests the same filter) or processing the cached version. In either case you eliminate the need for the upload, and you speed up processing server-side.
Just my 2 cents.