So I have this vertical monitor bartop running MAME.  Since I have a 1080P screen there is a lot of unused space on the top and bottom.  I wanted to have the marquee displayed at the top and the play area on the bottom part.
  I can edit the artwork .PNG files fine but I don't understand how to get the game screen to display in the right spot and size.   I know it has something to do with "bounds" but I don't know what numbers to put where.  Is there some kind of layout editor or painting app that works well to do this?  I know the screens are usually 3:4 or 4:3 depending on the game.   I know I need to put some pixel values into it but I am not sure what they are and painting apps are not set up for that kind of coordinate thing (at least the ones I tried).  What do you guys use to do this?