Then I covered each one with clear sticky backed plastic to make it more sturdy (I should buy shares in the stuff!).
Then I cut them up into different shapes. I discovered that the simpler the better as they can be a bit fiddly when they don't stick together very well and they tend to slip about.
You could also use larger pictures from magazines, greetings cards, family photographs, spare artwork, a letter, a secret message to a family member...there are endless possibilities!