@Orionsangel RetroArch MAME uses artworks the same way as MAME standalone.
Put the artwork.zip in system/mame/artwork/
and the config file in /saves/mame/cfg/
and you are fine.
(although no curvature or mame hlsl effects are possible - RA shaders will affect the artwork as well)
In RA the only problem with this setup is the separate config file as it seems to do something with the resolution.
The better way would be adjusting the settings directly in the *.lay file as i explained on september, 25 in this thread.
Since RA 1.8.0 there is a Video Layout
setting as well, that let you load MAME artworks as overlays with all the blend modes necessary. This way you can use the artwork for all systems/games you want (works only without the second config file)
Shader are working with this method, but only gl
driver is supported atm