Choose the "Composite" tab.
Enter the url for the pattern you want to fill the blank with.
You can use anything - animated or static, gif or jpg.
If the pattern you're using is smaller then the blank you're filling, you'll need to check the option for "tile across and down the image canvas".
Choose "IN" as the composite method and use center gravity. Leave everything else the same.
Press the "Composite" button.
That's it! Choose the "Output" tab and save your image.
 |
+ |  |
= |  |
 |
+ |  |
= |  |