I got this idea from Photjojo's newsletter - it's a great website that has lots of ideas of what to do with photos (http://photojojo.com/). I have about a million prints of nature photos. Thank God I have a digital camera at my disposable now. I have spent good money developing entire rolls of film with nothing on them but leaves, trees, and flowers. Half of them are blurry or just not as interesting as they seemed at the time.
Anyway, all you need is some photos (ones with bold colors or patterns seem to work best) a hole punch, and brass fasteners like these http://en.wikipedia.org/wiki/Brass_fastener
First, cut your picture into strips of equal width. If you want a bigger ball then cut the long way (you'll probably want two photos' worth of strips to make one ball). For a smaller ball cut shorter strips the other way (shown in the top picture); you should be able to make one ball from one picture for the smaller size. I cut the picture into six or eight strips for the ones I made.
Next step is to stack your strips and punch a hole at each end of the strip. Put the fasteners through the holes in your stack - one at the top and one at the bottom. Spread the arms of the fasteners in back to secure the strips together. The last step is to bend the stack of strips into a half circle and spread the strips out to make a ball shape. If you want to hang them on your tree, just attach a string to one of the fasteners. Needless to say, you could use other materials for this project. I've used cardstock with good results. It does need to be made of a sturdy paper, though, for the ball to hold its shape.