As it stands you can only report an image, you can't specify the reason for reporting it. Right now if I go to download a game that is very story heavy like an RPG there is a good chance that I could stumble across a spoiler for that game just looking at the images. Now idk about you but I don't think you should have to worry about that when looking at images to decide if the game looks interesting. When I report such an image and a mod comes across it without having played it, chances are they won't see the problem. If we had the option to write some additional notes with our report then we could specify exactly what is wrong with that image. And it's not just spoiler images this would help to report, imagine if someone with some modicum of skill in image editing went in and edited an actual game image to make it look like the actual game, you wouldn't be able to tell the difference unless you played it through. There can be other situational instances where you need more than just a simple report button as well.

I have one example of such a problem image on the Chrono Trigger(Snes) page. The first image you see is a spoiler from the end of the game
Spoiler warning:
It reveals Lavos the final boss' second to last form and gives hints as to what "Lavos" actually is (an alien) which is revealed MUCH later in the game. At first it is made to look like it originates from the earth itself and related to the monster races like an evil god.