Checkout Pixlr.  If you want to simply retouch your images but don’t want to buy Photoshop.  In minutes you can upload your picture and modify.

Here are somethings I would use it for:

- Adding text to images (like your website name)

- Reducing the image size

- Cropping out the background

Related posts:

1. How do I use customized CSS?
2. Stripe It Up, Stripe It Down, Stripe it all Around
3. Multiple Images
4. a clever way to supersize your images
5. 7 Tips on taking Product Pictures so you can sell more