It's usually a format thing. Mala prefers 24-bit PNGs. I've had to open pics in editing software and resave them as 24-bit before (usually resize them to the resolution I'm running Mala at for good measure).
If these are already finished layouts someone uploaded though, then it probably isn't a format issue.
Just to be clear, you're saying that it doesn't even show up in Layout Editor, correct?
If it shows up in layout editor, but not when Mala is running, it's usually a path issue.
When having Mala switch layouts based on gamelist, the layout name and folder the pic is in need to be formatted like: emulator_gamelist
ex. mame_beatemups
The wiki is here:
http://wiki.arcadecontrols.com/wiki/Mala_Layout