I use a couple programs that cost money.. however a friend reccommended gimp to me. I haven't downloaded it since I don't need it, and my computer's slow enough as it is.. But I trust the person who reccommended it. So here it is:
Just be careful, because I used a photo change thing on my computer once and I edited a picture to where it was b&w and then that is how the picture stayed. I ruined the only copy I had of a pic on my computer, on several occasions because I didnt find it out til later. So always try and save 2 copies. But I don't know how that one works
Try www.download.com and select a free download for mac or windows (scroll bar). Use the help button on each software to make the first steps. Make first a safety copy of your picture or pictures and then try the software. Sometimes you can download trial versions (free for 30 days) but be careful you cannot safe your work :( . Hope this help.