There are others of these, but I came across DrPic.com today in 5th period when Irena astutely asked if there are any programs on our computers that can edit images. DrPic.com does it through the web browser. Very cool.
You can do all of these things (shown to the right): resize, crop, rotate, add text, add frames, etc. Very cool and pretty easy to use.
If you have a very dark image, you can increase the brightness. Or vice versa, you could darken a very light image.