The Gravatar Java Sample Code by Gravatar demonstrates how to create an avatar URL with string hash and string email. For additional resources, consult the Java library available at https://github.com/ralfebert/jgravatar
Image processing probably isn't the most exhilarating part of creating a website, and a tool that could take care of all of that would definitely be appealing. Blitline is a cloud based image processing platform that works on a pay-as-you-go basis. The Blitline API allows users to submit thousands of jobs to be processed and pushed to their online photo storage service, using a simple JSON interface.
NASA has released a new API (the EPIC API) which provides programmatic access to images pulled from the EPIC camera. EPIC sits upon the DSCOVR satellite and constantly provides images from the Earth's sunlit side. Developers can retrieve images from the camera along with associated metadata.