A few years ago, I was tasked with rewriting an image processing service. To tell whether my new service was creating the same output as the old given an image and one or more transforms (resize, make a circular crop, change formats, etc.), I had to inspect the images myself. Clearly I needed to automate this, but I could find no existing Python library that simply told me how different two images were on a per-pixel basis. Hence diffimg, which can give you a difference ratio/percentage, or generate a diff image (check out the readme to see an example).
The initial implementation was in Python (the language I’m most comfortable in), with the heavy lifting done by Pillow. It’s usable as a library or a command line tool. The actual meat of the program is very small, only a few dozen lines, thanks to Pillow. Not a lot of effort went into building this tool (xkcd was right, there’s a Python module for nearly everything), but it’s at least been useful for a few dozen people other than myself.