Hi Judy,
I also use GIMP. It's a pretty powerful program and I have yet to learn even half of what it can do. The problem is, there *is* a learning curve and I had given up learning how to use it in the past because I found it so difficult. My need for an Etsy banner kind of forced me into learning the basics.
Etsy Banner article:
http://www.etsy.com/blog/en/2007/skill-share-making-a-banner/What you will need to do is import a photo into GIMP. When you open GIMP, you should get a screen. Click File > Open and open up the photo. Your photo must be at least 760 pixels wide (or tall if you plan to rotate it around)!
You then take out a *slice* of the photo that is 760 pixels wide by 100 pixels tall. You can do this by using the Rectangle Select Tool. You can also do this by opening a new 760 x 100 pixel blank window and opening a photo into this. Then rotate the photo around until you are seeing the portion of the photo you want to be your banner. If your photo is huge (pixel-wise) to begin with, rescale it and then zoom in.
If you want your banner to have words and designs, you'll have to open a 760 x 100 window and add in those things. For my banner in my shop (almondcrafts.etsy.com), I took a bunch of photos of buttons and things, edited and resized each photo, and then carefully added them layer by layer. This involved a lot of small steps, and despite the simplicity of my banner, took me a while to do.
When I save, typically save as a .jpg.
If I just confused you, there are probably a lot of tutorials on YouTube!
Good luck!