Maybe this will help. This is how I do it.
You create a new picture. I use 640x480, but it doesn't matter. You use "Image-Canvas Size" to resize the image to 450x125. You have to deselect the little link thing to make these values independent.
Border: The stroke function in my version of the Gimp always crashes, so I have to make my border manually. Make sure you save everything along the way, because the program crashes sometimes. Save as .xcf files and export them to .JPG or .GIF or whatever when you are finished. To make a border: click the pencil, then choose color, then choose draw size/type (ie. circle 3 or pixel). At this point I don't remember how to draw a line (it has to do with the middle button), but you can draw a very short one manually and copy and past to make the border. After the whole drawing is bordered, select all, copy, paste, Scale Layer: 446x121. This will create the inner border. Make the center transparent by selecting it (select continuous regions) and using "Layer - Transparency - Color to Alpha". This makes the center look like a checkerboard.
Layers: This is the most important part of this process. "Layer - New." Call it Picture 1 or something. Open another picture and select part of the image to paste into the signature. Next, choose "Layer-Stack-Lower Layer." This should put your image inside the signature.
Create a new layer for another picture or text. You can keep pasting and scaling like this. It makes it easier to put everything in separate layers because then you can just move around that layer and leave the rest untouched. You can choose whicy layer you're working with with Page Up/ Page Down.
This should help get you started.
