<story content="optional reading">
I first saw this referred to as a KPT effect called Canvas or something like that. I don't know what KPT package it's in, but when I first saw it in a magazine I almost said "HEY! That's a bump-map!" out loud. When you start saying things like that out loud, it's time to stop using GIMP for a while and time to step outside and look at the "real world" for a while.
Many of you are probably saying "Real world? Where can I get that plug-in?"
right now. Seek help immediately!
The trickiest part is creating a bump-map suitable for this purpose. Let's start with a blank image and Add Noise to it at 70 (Note: this blank image should be the same size as the image you want to canvas-ify, in this case 200x200):
1) The start of our bump-map
Now we want to create two Duplicates of this image. Gaussian Blur both of these duplicates at about 9.0, but do one of them only in the Vertical and one of them only in the Horizontal. Then Multiply these two together to get the following:
2) The canvas takes shape
This is not too bad, but we want something darker and a bit rougher...the way I achieved this (and you may find a better way) was to Contrast Autostretch image 2 and to Add Noise again at 20 to produce:
3) Our final bump-map
Now, we can apply 3 as a bump map to this:
4) A good test image
and it will produce this:
5) Our image has been canvas-ified!
As you can see, the final image looks very much like it's printed/painted on a canvas, or a canvas-textured substance. There's a lot more you can do with this tip; I leave it up to your imagination. If you do manage to produce something incredibly awesome, be sure to let me know. Thanks!