You must flatten the image and save it, I recommend the .png file format for the highest quality.
You then will need to upload the image to an image hosting service, such as photobucket, imageshack, or tinypic.
This is covered in the bottom section of the thread:
GIMP, short for GNU Image Manipulation Program. It's a program that is popular for many uses, there are a lot of tools available for use in GIMP, you can complete image manipulations in GIMP, and other smaller projects, it's not meant for just one purpose.
Have a good one.