For either a background or CG, would it be within the rules to do some very simple edits, such as changing saturation or contrast of the whole image? I'm planning to have only a picture of a landscape (either used as a CG or background depending on which fits the rules better) and e.g. desaturating the image or upping the contrast to neon levels would be sort of analogous to changing sprite expressions (since the landscape will effectively be my "character" here). I don't expect any of these edits to take more than 5 minutes to do since it'd just be opening the image in an art program and moving some sliders around.
If that would be acceptable, how about slightly more detailed edits such as turning part of the image to black and white while the rest stays in colour or adding effects such as blood splashes on top? Would things such as changing weather between rain/sunshine etc be ok? (I assume weather is going too far in terms of changing the setting, but I figured I'd ask while I'm still at the brainstorming stage)