This Tip comes from the twisted mind of Jonathan Thompson
Creator of CerealKillers Help Desk

The image we're starting with here (and, feel free to snag that image and
do this example along with me) is 100pixels by 100pixels. For this technique, we create a new image that is twice the size of the original (200pixels by 200pixels), with the same resolution (150), same color mode (RGB), and a transparent background. If you're starting with a gif, it might be wise to change it's mode from Indexed to RGB to help stop color shifts and *blocky* pixels.
Start by dropping your tile in the bottom right corner. It doesn't always have
to be this corner, but you'll have to change the steps and placements of the next layers to make it work. For now, just use the bottom right corner. To *drop* the tile, click on the move tool, then click on the image itself... drag that image over to your new project until you see the edges of the new project light up. You could also copy and paste it there, but this is easier

Make 3 copies of this layer, so that now you have 4 layers with the same thing.

Move the first copy up to the top right corner.
Go to Layer>>Transform>>Flip Vertical. This will set the whole right side.
If it isn't completely in place, don't worry about that now. We can fix it later =]
Now go to the second copy, and move it to the top left side.
Go to Layer>Transform>Rotate 180°.
Go to the last copy (3rd), and move it over to the bottom left corner.
Layer>Transform>Flip Horizontal. This will finish up the seamless tile, but
if your blocks aren't quite touching each other, the easiest way to fix this is to click the move tool (you should already have that one selected), then use the little arrow keys on your keyboard to nudge it into place. Here's what I made out of the first image: