For this specific case I removed the image because it wasn't relevant at all and to be honest I don't even understand how it was meant to be humorous: it was just a bridge with some buildings on. It didn't add anything to the question and only really, in my opinion, detracted from it. I also think your title here is a bit misleading, humorous is subjective, but it wasn't really a part of the question, never mind a non-essential part: it was just a picture of a semi-relevant modern bridge.
I just want to add a brief side note here, you say (emphasis mine):
Now, obviously there's no need for the image of that bridge. But I like the question better that way, and it got two upvotes and many interesting answers (not just mine...)
Now those upvotes and answers may have come because of the image of the bridge but I highly doubt it. Adding that section in the question here makes it read like they are in support of the image when there's no indication of either way and I do doubt that to be the case to be honest.
Now in the general case of if it's fine to keep humorous but non-essential parts of a question. Sure, of course it is. We don't hate fun here, what would be the point otherwise? But there's a difference between adding a picture of a random bridge/meme and adding some funny commentary that is actually somewhat relevant. I, myself, have done the latter a few times around the site and it's done quite a lot round here. But I think you have to make it apply at least a little bit to make it work.
And lastly, it's always worth noting that it is still your post to do with as you please. If you want to keep something as it was, it is always your right to have it that way (excluding Code of Conduct breaking content, spam, etc. of course).