

Honestly I think the easiest thing would be to not allow images or embedding at all in PMs and perhaps display a warning message when clicking links “you are leaving [instance name]…”
Analyzing potentially lots of text and images in an effort to “guarantee” safety of users is likely a sisyphusian endeavour that is bound to fail - and furthermore also has privacy issues (namely that “private” messages aren’t private at all)
Here’s a few reasons:
Not funny
Not nuanced
Bad art
Comes off as smug/condescending (author)
Makes fun of/disparages broad groups of people
Assigns positions that people don’t agree with to labels they identify with.
Etc. Etc.