An easy service for you to edit your photos online can be found at PXN8 (Pixenate).
1. Click on the "Choose Your Image to Edit". Navigate to the file you want on your computer and double click or press "Open" to upload.
2. The service will say "Uploading image, please wait", then it will open the editing tool. Resize, crop, enhance as you wish. I love the "fun effects" option. Note that there are UNDO and REDO buttons if you try something out and you don't like the result.
3. Click on "Save" when you're happy with your picture. This will open a download window (it automatically formats your pic as JPG) so you can save it to your PC.
A simpler service is also available at Snipshot.com.
Other similar services offering different capabilities are Cellsea, Online Image Editor and Fotocrib.