I use Gimp as well.... it's free! Here's a short tutorial on how to add a watermark: http://www.simplehelp.net/2008/12/12...ge-using-gimp/
I save my watermark as a separate image with a low opacity and then copy and paste it to my images. I sometimes have to change the size of the watermark depending on how large or small I want it on my image. Best of luck! It takes practice to get good at any photo editing software!