So to reiterate the current issue here, there is not a way to set a default image in Mala. One so that for every rom, the info window would display the same image. Currently the only way to do that is to rename the same image, one for each rom name. The problem is that's pretty tedious, as we needed to do it manually (as danny_galaga found out). Well I broke down tonight and created a little app to do this. It's pretty self explanatory. Just takes a source image, you point to a rom folder and specify whatever file ext your roms use, and it will split out the same image, but renamed for each rom.
I know there are more pressing fixes in Mala so until then you can use this. It's written in .NET 2.0, so if you don't have the framework installed, you'll need it first to run.
Good luck!