When I had my cab running for some testing I figured this out.
I created a empty folder to store my snes snaps in and set it up in maximus's options/setup screen. I had F12 setup to take snaps when pressed. I found all I had to do was start the game through maximus then press the F12 button to take a snap. It would automatically save the snap in the snes's snaps folder and display it on the screen in Maximus. (I think it only did this for the first snap taken of whichever snes rom you are playing).
If you allready have the snaps and know what game they go to just name them the same name as the rom.
Otherwise I have no clue.