I made a wallpaper but I was wondering how you resize it. For instance, sites have wallpapers that have sizes such as 1280x1024 and so on.;)

Well in photoshop, u just make a new document, pic the size (800x600; 1024x768, etc.), paste the wallpaper you want to resize, go to edit->transform->scale, and either condense it or stretch it out to fit inside the boundaries, and save it. Ta-da! XD