Once you made the image in your paint program simply copy it, then open traxx and go to 'Edit/Track Components' and right click on the icon bitmap you see there and select paste from the faz menu, save and your done.
Note that mtm2 will only display a few custom bitmaps so if you don't see yours in the game then move your track higher or to the top of the pod.ini list and then you'll see it.
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Here's a link with a more detailed explanation, and a how to use stock bitmaps...
http://cownap.com/~mtmg/traxx/component.html
