I don't know it either, but luckily I printed it:
create your normal image
select and copy it (Ctrl-A, Ctrl-C)
select Image - Image size and manualy enlarge the image with 100 pixels
Do filters - blur - Gaussian Blur - 50%
Do Ctrl-V to paste the original image into the blurred frame
select file - new - and enlarge the new blank image with 10 pixels vertically and horizontally
use the paintbucket to color the new frame in the desired color
Ctrl-A - Ctrl-C
Go to the photo file
Press Ctrl-V
Drag the photo layer to the top of the stack in the layers menu
Select layers - Flatter Image
and that should do it...
Best regards,
Walter