Exactly how plagiarised constitutes plagiarism? Same image + same text is easy enough, but what about cases in between?

There’s a line I’m pretty sure I’ve invented rather than remembered (can’t find anything similar via Google), that will work as a funny subtitle for a variety of pictures, but under a different main title each time (depending on the picture).

Different picture, different title, same subtitle = self-plagiarism? Or not?

Thinking about it, there are some jokes where much the same joke will genuinely work for more than one picture (not in the vein of a “Pic unrelated” joke, just that more than one image fits the bill). So then it’d be the same text, but under a different picture. Too similar?