The International Image Interoperability Framework (IIIF) has put a draft API for the delivery of images via a standard http request. It supports information requests as JSON or XML as well as image requests.
One of my first reactions is that it sticks to the letter of RESTful interfaces while doing things that would be more sensibly be expressed by URL parameters. The following are offered as example URLs:
That’s harder to understand than something like x=80&y=15&w=60&h=75.
A service must specify the level of compliance it provides, which may be different for different images; for instance, JPEG2000 images might be scalable but GIF images not.
If widely adopted, this API could simplify access to images spread across multiple repositories. I'll be looking at it more carefully as soon as I find the time.