I recently used one of my photographs to make a set of stickers. I cropped out the bits that I wanted to use in the design, adjusted the colors, resized the image, and took it from there. I use U.S. letter-size glossy sticker paper, but A4 size would also work. These stickers are small, and 25 stickers fit on each sheet of sticker paper. After I print the sheets, I cut the stickers apart --- that's the only tedious part of the process.
I love how these stickers turned out, and have used about 200 of them so far in various mail art projects.