Update #2 -
I figured out that if you drop a hi-res marquee down to 739 wide, mGalaxy 6 will leave it alone if the height is over 207 pixels. If the resulting height is above 207, the marquee displays fine.
Nope. Totally wrong w/ the above comment. (FYI, the following is what I found from my testing. Not a clue what the code is actually doing. I believe I'm write in theory, but my assumption as to what mGalaxy is doing to get there may be different)
mGalaxy wants the marquee in that 739x207 box....and it seems like it'll adjust the height to get there. The logic behind it seems to be, "resize to fit, but stop if the resizing will give you black bars". So unless you've put the full artwork of the marquee into a 739x207 box, you're going to lose some of it to stretching. (for rectangular ones anyway, didn't even start on oddball shapes)
For example...
I grabbed a marquee at 1440x374. Since mGalaxy just wants to fit the box, it drops the height to 207 so it fits. That leaves the width over 739, and I got a "stretched" look.
Conversely, I used a marquee that was too small. 500x110. mGalaxy raised the height to 207, but it keeps the aspect ratio, so the width went up too. Too far actually, so a tiny marquee looked stretched.
So in both scenarios you're left w/ a width that's over 739....and a stretched looking marquee. Some worse than others, but always stretched.
The only marquee that's not for me is Crystal Castles but that's because it has a big black border. So when the marquee was fit into the box, the "too wide" image looks fine because only black is being chopped off the sides.
So if you want the full marquee artwork, you have 2 options. 1) Wait for some future toggle that'll give you all the art, but black bars. or 2) Customize everything yourself
Update #3 - Customizing marquees. I'm going to be customizing mine, as I have an idea for marquees that wind up giving me horizontal black bars. Below is my creative idea to deal with it. I just put the black bar on the bottom only, then use that space as you see in the image.
For marquees that give me vertical bars....nothing I tried creatively looked good, so I'm just going to roll with centering the marquee and having left/right letterboxing.