Well, as a first step I thought that if we have a site with changing content, like a blog, then the area of blog posts occupies some percentage of the site. So we can correct the threshold, like, if the picture has changed more than 50% then alarm.
Regarding static sites: well yes, they are easy to protect, however I guess there are tons of sites on wordpress with 1-2 pages (like company site), which are in fact static but written in php and may use outdated plugin or something.
I'm somehow thrilled of this idea because I want to provide something new, while it's pretty hard :)
In the end it's kind of possible to do all this stuff like - Select areas which should be checked for changes - Select areas which shouldn't - As you said, text-basic machine learning, though it seems hard for me, but still - Plus, what I also like is Google Vision API which at least can detect adult content. This should be pretty universal, if you know that you shouldn't have this on any condition, even if you have dynamic content changing
Hmm..