@GemaH: not so easily. Mame artworks supports blend modes, which overlays doesn’t.
blend modes
____
1. layer: bezel (blend mode: normal) \____\ ══╗
------------------------------------------ ____ ║
2. layer: backdrop (blend mode: screen) \____\ ══╣
------------------------------------------ ____ ║
3. layer: overlay (blend mode: multiply) \____\ ══╣
------------------------------------------ ____ ║
4. layer: game screen \____\ ══╝
Also it involves a bit of image manipulation. Mame artworks can have parts of the image that go over the monitor screen or are smaller, but overlays stretch the image to either completly fit your monitor screen (if overlay0_full_screen = true) or the hole game screen (if overlay0_full_screen = false)
To maintain the aspect ratio you have to resize the images (cropping or adding borders)
In the case of Orionangels artworks, i think they are all in 16:9 ar, so no clipping is needed, but if your monitor is 4:3 or 16:10, than you have to do some work…
For the overlay, mask, backdrop elements you have to tinkering a lot, to make them look like in the artworks.