Okay, I would suggest moving it to photoshop and making it transparent. An easy way of doing this is removing the back ground layer by cutting the image out of the background, closing the window and opening up a new canvas.
Paste the image onto the canvas.
Go to your 'layers' window which, if you cannot find it, you should be able to locate it on the top menu by going to Window --> Layers
Go to your layers window and then go to the 'channels' tab. Click the thing that looks like a hollow circle, it's very small and it's at the very bottom of the channels tab/layers window.
This will select the entire image.
Then, once this is done..
Go to: Select --> Inverse
This will select only the drawn image (the tablet-drawn image).
Okay, now this part is a little difficult so bear with me if you don't understand:
Make sure you have the color black selected in your color pallet, otherwise this won't come out correctly.
Create a new layer and when you open this new layer, select it.
Hit Shift + F5 and this will fill your image onto this new layer.
Resize it to the proper proportions, then save it to "save for web devices" as a transparent GIF.
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So far I've only tried this with non-computer images, but it should work with computer drawn images too.
Keep in mind that this can be used in GIMP or Photoshop CS3. I haven't tried this in GIMP yet, but I intend to experiment once my laptop comes back from service.
I found this all out through a tutorial on the web. Once I'm able to get that from my laptop too, I'll post it for you Mr. Mystery.
@Chewett: I'm not 100% certain about this.. but I think you can create computer-created images so long as they stick to the MD style of art. It would be something to ask Mur.. otherwise I'm not certain.