Stretching the layout pics also results in stretching info-window pics. Can't help you with that but if someone else has a "workaround" for that I'd love to hear it.
What you could do for your layout (snaps showing behind the flyer-problem) without using stretch, so images in the info window are "normal", is
a) make a copy of your snaps in a new folder and then copy the flyers to that folder overwriting existing images
b) point mala to that new folder as for example being "snap/title", also make sure in your layout there's only a snap/title window
c) to still be able to see snaps
and flyers of a game in the info window, you go to your mala option and change the artwork directories (eg snap/title=the new folder, marquee pic=the original flyer folder, cpanel pic=the original snap folder) if it is for mame you're using those flyers, you could ofc use for example definable 1,2,3... instead of cpanel pic or marquee pic
d) go to mala options -> GUI -> info window. Change Snap entry (type=image, image type=cpanel or def1...whatever you took, title=Snap) and add if needed flyer (type=image, image type=marquee or def2...whatever you took, title=Flyer). Be aware doing this for other emu's then mame means you'll have to change the image directories for those other emu's 2
Hope this helps

You could also photoshop every single flyer, giving it a border
