These are the online image tools I have found most useful…

• Snipshot – for simple resizing and cropping it is a breeze to use.
• PXN8 – some useful functions, including a spirit level tool.
• Phixr – a good photo editor, which also allows you to email the photos to friends which could prove useful.
  
• ImageAuthor – not so easy to crop to specific areas, but has some good basic paintbrush tools, including a colour picker!

There are lots more online image editors, a longer list and a full comparison can be found on the Smiley Cat web design blog.