I have this in my canvas:
What I want to do is cover the whole canvas with this image; the image is 250 px wide, so I want to add 3 more so the whole canvas looks like fire!
How do I do that? (I've read the GIMP book, looked on SO and Google, but found nothing...)
Select and copy the image to put it on your clipboard, then use the bucket fill tool with the pattern option, selecting the clipboard pattern. If you want to keep the image around for further use, you can always save it as a pattern.