Thanks guys!
It took me a little to figure it out at first myself, as I learned editing software from PS, but when you open an image to resize it, there should be a button that looks like a few links in a chain off on the right (usually beside the image size or resolution bars). By default, they look like they're seperated. If you click it, that locks the aspect ratio in place, and you can resize away. Alternatively, there should be options in the Peppermint Settings Panel. Try automatic, or zoomed fill.
It also looks like you're using a crazy widescreen display. Do you know your aspect ratio off the top of your head? I usually make packs of wallpapers with different sizes/ratios, so I can make some of those up for Peppermint.
I'll make a thread with a bunch of the ones I've made, but they're usually non-OS specific. Movies, music, games, and that sort of thing... Here's one of my personal favourites that other people seem to love just as much!
Spoiler (click here to view / hide)
Ohh! I like those! Especially the first one!