A few people have asked how to do this, and it's just plain cool like that. Rayhawk didn't specify that this forum was limited to finished artwork, so here goes...
NOTE: This tutorial is geared towards GIMP.
PS users might need to find their own versions of various dialogs.
So, first, you have to get an image. I'm using the one below, which I drew myself, but you can just use any old thing.
Official Step One: merge everything down to one layer. You can do this by selecting the Layers dialog (open/focus it by going to Dialogs > Layers in your image window), right-clicking, and then pressing W (for Merge Dow
n) for every layer that needs to be, er, merged down. You should just have a Background layer.
Step two: copy the layer and name it Bloom. Right-click in the image and select Colors > Contrast, and bump up the contrast by positive 30 (the bottom slider, moving towards the right). I got this:
Next, you want to right-click again, picking Colors > Curves. Add one point at about (127, 0):
And I ended up at:
Now, right-click on the Bloom layer in the Layers dialog, and select Add Layer Mask...from the next window, Grayscale Copy of Layer. Now just right-click and select Apply Layer Mask. I got...
Now just right-click on the image, and select Filters > Blur > Gaussian Blur. Enter a value of around 8 into both boxes, then just click Apply. There you go! I suggest changing the opacity of the Bloom layer to somewhere between 20% and 80%. Here's my final image:
Postscript: The brighter and more bad a color is, the more it will be blurred around everything. For example, take a look at my sig. The bad bleeds into everything else pretty well, while the semi-bright blue in the drawing doesn't spread much, but the bright yellow does.