GIMP is the GNU Image Manipulation Program. It is a freely distributed piece of software for such tasks as photo retouching, image composition and image authoring. It can do almost anything that the commercial package "Photoshop" can do, but best of all, "The Gimp" is FREE. That's right, it is open source software that you can download for free and use immediately.
The GIMP is just a bit confusing when you first begin to use it, as the user menu "floats" on your desktop and is not attached to the image. Whichever image is "on top" is the current image. The user menu will look like this:
The image you are working on will appear in a box like the one below. Once you load your image, you can choose which file extension to name it and simply save it with the new name. The Gimp automatically converts it.
Compressing a jpg image is easy with the gimp, when you save an image as a jpg, a dialog appears which then asks you how much compression you would like to apply. This makes your image sizes smaller and easier to email or helps to conserve bandwidth and speeds loading time for your blog or webpage.
Some other Tutorial Sites:
You can get a large number of add-ons and plugins for the Gimp. Once you have mastered its basic functionality, you might be ready to try some advanced image editing/creation techniques. You can find many of them at either the gimp.org website or on Sourceforge.