Should we allow “From which episode is this image?” questions: No, too broad
Should we allow "From which episode is ....?" questions in general: Debatable
"From which episode is this image" questions become too broad fairly easily. Unless a reverse image search quickly manages to exclude the image from being from a specific movie, or visual novel.
If that is not the case, it would mean that the person answering would need to have a very in-depth knowledge of the source material, which sometimes includes more than thousands of hours of screen time. Or they have to go re-watch/read the whole thing just to get this one answer, which in most cases will not help a lot of people either way. This is based on some of the "from which episode" questions we already have on the site.
Then should we just disallow "from which episode" questions all together?
No, I don't think that is a good option. Most image identifications become too broad fairly fast. And even though this can also be the case for a bulk of the other "from which episode" questions out there, they could be borderline answerable. Taking this K-on question and this One Piece question, which are also fairly broad, yet are not bad questions in themselves.
In both cases, I think the questions should at the very least suffice to the
ID-request rules in general, even if the identification-request tag may not always apply.