Ask HN: What “APIs” return images based on query strings

I'm working on the Salesforce platform, and one of the few interesting things that fairly non-technical users can do is create excel style formulas, including formulas that can display images.

I'm looking for web services that return images from GET requests. Some examples are Google's (deprecated but still useful) charts API[0], and the incredibly useful Da Button Factory[1].

What other websites/webservices/"APIs" out there can display images based on dynamic query params?

[0]:https://developers.google.com/chart/image/ [1]:http://dabuttonfactory.com/