PicMonkey.com
Photo editing online.
The video to the right shows you how to crop your images for your website.
It covers how to crop a portion of an image out and how to resize an image smaller and then crop it.
The default settings should be set to save your images as a jpg format, be sure to keep that unchanged. The medium size is option when saving is good for your website, you want to keep your load times down.
PicMonkey has some great tutorials showing you how to use the features in their website, check them out.