Pictures should be used to convey some information relevant to the post. For an example see What is fairly new theorem one can teach (and prove) to an undergraduate student?
Pictures should not be used, at least not on a regular basis, for aspects tagential to the post (such as illustrating a cultural reference), to just make the post look nicer or "for fun."
Put differently, in my opinion, everybody should feel free to support visually the content of their posts. However, I would prefer if such illustration is (in general) limited to the content and not side-remarks.
As always there can be exceptions, only I do not want this site to turn into pages full of pictures not really related to the content. On the one hand, I think this might send a wrong message regarding the purpose of the site, and on the other hand it can be inconvenient depending on device and the connection one has available.