"Image" in Esperanto is also best described by the word "bildo".
Unless you are talking about image is in a rock star's image, or in the sense of an idea that you imagine.
"Imago" in Esperanto means "imagination", or also "something imagined", but you will see many use it wrongly to mean "image" where they should be using "bildo".
While “bildo” is usually the word you would use when translating "image” from English into Esperanto, in this particular context, “image” isn’t what one would normally use in English. Like many “synonyms” they don’t mean the exact same thing, even if they do overlap.
In English, “to paint” goes with “picture” or “painting”. That’s not to say “image” isn’t possible, but the choice would and should communicate something particular about either what is painted or how it is painted, and in most cases, it would be some kind of metaphorical painting going on.
The part of what you're saying that I agree with is that "to paint" usually goes with "picture" or "painting." It is possible, however, to "paint an image" or "to portray an image of someone in painting" - in which case you might say "figuro" here. So if you're saying that the translation has to be "picture" here (not "image") then ultimately I agree.
However, I would hesitate to agree with your use of the word "usually" in your first sentence there. As I think you are also saying, it depends on context, and who can say what the "usual" context is?
Good point. I didn't consider the sense of "portraying an image of someone/something in a painting", possibly because this particular context doesn't lend itself to that interpretation.
I suppose you are right in that it is hard to say what context is most "usual". Context is context is context. And without it, learning languages would be easy but boring.