I usually have trouble getting the sizing right, but with this software it was so easy. Here is how I did it (I use windows):
Open GIMP, Click 'File', then 'New', and there you can select your image size. Click OK
Next, open your Pictures folder which contains the image you want to use. Drag this image into GIMP.
Select the 'move' tool in the GIMP toolbox (a cross with four arrows). This will allow you to drag the image around, so that the preferred section of your image is shown.
Then it's time to add your text. Click the 'A' in the GIMP toolbox, then click on the image where you would like the text to go, and type your text. You can select your font, size, colour etc in the bottom section of the toolbox.
When you have your text in the right place, and you are happy with the way your banner looks, click 'file', then 'save as' and give it a name. Also you may have to change the file type to a jpeg before clicking save.
I hope that makes sense! Here is mine. Next thing I need to work out is how to give my text a shadow so that it stands out a little better against the pale section of my pic.