Actually that is correct, but you will want to use it in a transition callback, something like:
local emuImage = fe.add_image("attract.png", 0, 0, 100, 100);
function transition( ttype, var, transition_time )
{
if ( ttype == Transition.StartLayout || ttype == Transition.ToNewSelection || ttype == Transition.ToNewList )
{
local current = fe.game_info(Info.Emulator);
emuImage.file_name = current + ".png";
}
return false;
}
FYI though - if you compile a recent build, I believe you can use the "magic tokens" directly in the filename you pass, so you can just do an fe.add_image("systems/[Emulator].png"); and it should update although I haven't tested that yet.