Likely not so as you can tell. Whatever your picture size, and whether you fill screen, fit to screen, fill screen, stretch to fit or tile, somewhere along the way, you mac is going to create and store in an easily accessible place, an image that covers your screen neatly. That easily accessible place is likely in graphics RAM, rather than in your desktop pictures folder. The conversion of your picture file into what's actually shown on screen likely happens only once in a while, each time you log in, and when you change your desktop picture.