I don't believe it would be copyrightable in the US
"If a pictorial, graphic or sculptural work is a useful article, it is copyrighted only if its aesthetic features are separable from its utilitarian features. A useful article is an article having an intrinsic utilitarian function that is not merely to portray the appearance of the article or to convey information. They must be separable from the functional aspect to be copyrighted."
However it's probably safest to assume that all roms are copyrightable as it avoids getting involves with layers.