The ability to do things on the web is expanding all the time and on-line photo editors are more or less coming of age now. They may not have all the features of Adobe Photoshop or Corel Paint Shop Pro (or even GIMP) but they probably have most of the things you are likely to want to use, albeit just occasionally. Here are three of the better free ones.
PixIr, this is a comprehensive photo editor created in flash. so you need a flash plug-in to use it. Whilst it does cover a range of handy photo manipulation tools it is probably the filters that will appeal to most.
Picnik, is available to edit photos within Flickr, it is really easy to follow and less like the current range of photo edit software with their incomprehensible toolbars. Picnik has tabs that are more like wizards and pull down pick list (just graphics options). The frames and and stickers (stick on clip art) are probably the most fun.
PiZap, is also easy to use and as it rates itself creativity wacky and the worlds EASIST Free onLine Photo editor. There are not as many features but it definitely for fun.
All these offer easy upload to a number of social networks, like Facebook, Flickr, Photobucket … etc. All have an optional upgrade or pro membership option so you can store your work online and/or get more editing options.