Disable -use_bezels, and maybe disable -use_backdrops and -use_overlays. Or delete the file in your artwork folder in mame folder. Or change -artpath to not point at the artwork folder.
More info on these options in mame's docs folder, config.txt.
Don't know how to disable? Which build are you using; mameUI was a different UI than mame, and it can be differently between the two.