I'm not sure about this, but I thought I've heard that SL uses alot of proprietary software.

And, as such... designing both avatars and full characters may not be able to be done using the more atypical, common image formats such as JPG, GIF, PNG, BMP, and others, for example.

In other words... unless there is a image editor program of some sort that outputs the designed and then uploaded image in the image format that the SL engine recognizes, then there is no compatibility with their proprietary software engine.

But, again... not totally sure on this, so don't hold me to it. *Shrugs*