"Spider on the ceiling" ... I like it, though it might be a bit subtle for my usual audiences.
Also in the "few fries short" category: "Not enough brains to wad a shotgun" may be a bit opaque for the non-shooters among us (which is probably most of the community), however, "He's got a whole lot of postage but he's missing the zip-code," should be more universally accessible -- I know Europeans don't really have zip-codes, but the idea translates ... doesn't it?.