well if your an windows user or an mac user an great alternaitve to adobe photoshop is gimp as its free and open source and runs on both windows and mac but i do like gimp shop more witch is gimp but made to look and fell more like photoshop its self but only for windows.
if you an windows user you might want to check out paint.net
and you also have clinepaint for mac as well
you also have services like picnik that are online but dont give you as much power as gimp witch would be my choice for both windows and mac over photoshop there are many more but thies are the ones i like