If the picture is 960 pixels high and wasn't resized upward from a lower resolution, it should be fine. It should look at least as good as it does on the computer.
540x960 is the Photon's/Electrify's actual resolution (qHD) but 1080 = 540*2, *2 being the width of your typical Android wallpaper, twice the width so the picture can shift as you change home screens. (Personally, I prefer to use 540x960 images and turn off shifting. If your launcher supports this -- GO does -- your wallpaper crop tool will have the proper aspect ratio. If it doesn't, it will try to take a piece with the aspect ratio of 1080x960 out of your 540x960 image.)
I resize wallpapers to 540x960 for use on my Electrify. I posted some in the wallpapers topic; if you want some nice ones (note that they do not support homescreen shifting) have a look
Samsung Galaxy S3 / US Cellular / d2usc