[L1Slayer]

Well technically it still is "illegal" because you are paying someone money, while he/she is using Artworks/Screenshots/Pictures that are subject to copyright. (making extra changes to the artwork doesn't help. It's still based on a copyright protected piece)
Basically, as soon as money flows and official material is involved, you need a license to sell it.
It's the same with statues. Recreating a character from a Trademark without permission is illegal.

But to be fair, I don't really see this as problematic since the market of custom steelbooks is very thin. And Publishers don't seem to care about it either.