I use a couple of JS libraries to serve image galleries, and they conform to the Flickr _[a-z] type of naming to indicate resolution. The library will load the appropriate size based on the available real estate.

The auto serving of the chosen output extension is interesting. Wonder if they were automatically created when making the various image sizes, or only on the first time it was requested. And how many formats were supported? The one that got me was hitting an API and tagging with xml|json to get the response as needed instead of sending it as a dedicated query paramter