There's always a reason you might want to edit some of your images. Maybe you want to create a watermark or crop something out, make something a bit more fancy or all sorts of things.
You'll want to experiment with it, but here's a basic tutorial.
Can't see the video? Watch it here.
Collages and more…
Update: Want to make collages? I created another tutorial using iPiccy for collages: How to Make a Photo Collage.
(If you use Picasa, you might also be interested in How to Make Photo Collages with Picasa at skiptomylou.com.)
Yet another alternative is PicMonkey which looks promising as well (with a collage feature to come soon).