Methods for deciding on cover art?

All of this is just my opinion from working in cover design.

It is extraordinarily rare that a good cover would be an illustration of a scene from the book. The image on your cover is an appeal to a demographic that says, "Hey, this book was written just for you!" Many of the most recognizable covers out there don't use a photo or an illustration for their designs, but it really depends on your genre and the kind of person you are trying to appeal to.

There are also successful covers that depict a scene that's not in the book. These covers work because they engender many of the same themes and ideas that are found throughout the novel. For example, a coming of age story about an all boys boarding school could be represented by a class photo even though there was never a class photo scene. Or a macabre novel with strong themes of death could be represented by a gravestone with a mirror set in the granite.

A good novel expresses a lot of different themes and ideas. A cover does the same thing in a more suggestive way. If you're going to use an illustration of a specific scene, it needs to be a tell-all scene that is compelling from an outside perspective (i.e., from someone who knows nothing about the story).

I'm not a pro cover designer. I just make them for my own stuff. But how I do it...

I don't think in terms of scenes. I think about the themes of the book and the sort of readers who might pick it up. I look at a whole bunch of covers aimed at similar readers, to see things like font choice, style of art, colour, layout, etc.

I think about cover ideas that use my strengths and avoid my weaknesses (there are no marks for effort if you attempt an ambitious cover and fail). I'm planning these several books ahead, so that I can gather up any items and texture photographs I might need. So though I currently have two covers done, I'm already doing the foundation work for several other covers.

When I'm contracted to do a cover I try to capture what I think the essence of the book is without capturing a specific scene, though I do sometimes borrow what I think are interesting elements from a scene or scenes. I'm most concerned with doing what I can to encourage a potential reader just pick up the book.