I had a recent requirement to add multiple copies of the same image to a larger sized image for printing. Effectively I needed to tile the background of the larger image with my smaller image without any gaps. As there were many copies of the smaller image to be aligned I didn't really feel like trying to manually align each of the smaller images (I have better ways of spending my time).
The simple solution was to save the small image as a pattern file with a .pat file extension in the "C:\Program Files\GIMP-2.0\share\gimp\2.0\patterns" directory.
In my case the image that I needed to tile was businesscard.pat. When saving Paterns you are prompted for a description. The description is used when applying a pattern from the "Bucket Fill" tool, so make sure that it is something meaningful. I used "NZDI Business Card" for mine.
You can now select the new pattern from the "Fill Tool" and apply to the larger image. You may need to refresh the list or restart the GIMP before the new pattern shows up in the list.
The more that you use the Gimp you will learn that there are always multiple ways of achieving the same result. What I could have done was to simply use the "Filters > Map >Tile" tool on my original image, this would have been just as effective. But, by saving as a pattern image in the "C:\Program Files\GIMP-2.0\share\gimp\2.0\patterns" directory you will always have the image available within Gimp to re-use with the "Bucket Fill" tool.