I've thought about this and asked some questions. By all means I'm not trying to undercut some of the fine people who can actually die-cut and make decent decals, but try this (for a cheap version).
Here in the US there's a paper called DAFC8. Its made by Chartpak. Its basically a clear sheet with ad adhesive backing that will fit into an everyday printer. Print a color copy of an image and you'd be able to cut it into a sticker. Now I havent actually tried this, but its almost certain to work somehow.
The guy I spoke with said the adhesive wont be super sticky, because anything heavy would gum up a printer. If/when I try it, I'll let you know.