I think the answer is "Yes, it's allowed", but if the answer consists only of the image, then the answer is likely to be downvoted as "Not useful", since the information can usually be presented textually.
If the image is part of an answer, and actually illustrates something — rather than being simply an example of laziness in not bothering to transcribe content — then I don't see a problem, myself.
Any image has to be relevant, made accessible to screen-readers (with the correct alt-text), correctly attributed and not a blatant breach of copyright (i.e. fair use applies, as an illustration of a point). Here's one I prepared earlier: I'm sure there must be other examples, too.
An image which is not relevant and which can be edited out without affecting the answer should not be included and can be justifiably removed by anyone with editing privileges. There aren't many examples of those because they've all been dealt with.