Actually I think the instances where this bot would be useful are the rare ones. Most images on github are probably for the project's logo or gh-pages branch. It's simply not important that those be compressed and getting pull requests on things that are not core to the project's purpose are distracting.

Yes. A bot could try to identify what kind of project it was (are there html and css files etc). That might be an interesting project in itself trying to classify github projects into libraries, web sites, documentation, standalone apps etc.

Those images don't sound big enough to trigger the bot. So we're back to it being usually useful.