My favorite site to find fresh wallpapers is InterfaceLIFT. It's a site where people like you and me submit our own pictures for everyone else to enjoy. All the wallpapers on the site are FREE and they're all high resolution. The site is like a cross between Photobucket and Facebook. The best part is that it lets you choose your resolution so that the wallpapers look crisp on your screen.
If you want to swap out your outdated wallpaper head over to www.InterfaceLIFT.com. If festivity is your thing, click HERE to see all the pictures tagged with "winter" on interfaceLIFT.
Your screen resolution is the "internal" size of your screen. You want any wallpaper you decide to use to match your resolution. You can go here to see what your screen resolution is.