Look for imagery that expresses the mood or overall tone of the magazine. You want to provide a context, not a literal image.
And remember to focus on one main image. It’s tempting for some authors to get carried away by trying to represent all the symbolism in their stories. But when it comes to magazine covers, less is definitely more. Remember that the cover will be typically seen as a small thumbnail or while the reader is walking by a retail shelf and you want your reader to understand what they’re seeing. You don’t want to confuse or overwhelm the reader with conflicting or busy imagery.
Good design is about balance. Pick one constant and tell it visually through the cover design.