This video demonstrates how to extract fine portions of an image while
maintaining the original image using a Layer Mask in GIMP 2.8 running on
Windows 7.
This is an old tutorial but it demonstrates how to create seamless textures for Second Life. A good watch, although the website linked to in the video no longer exists.
This is the latest video tutorial from the brand new SL-Inspiration YouTube Channel, which explains how to save and listen to a Second Life Music stream through a media player, and get much better quality sound.
This is a walk through tutorial from Claudia Dryke that has music instead of any speech for some reason, but it may be helpful in showing how useful both Makehuman and Avastar are for creators.
If you are considering creating your own poses or animations for Second Life, then this tutorial by Medhue will go a long way to starting you off. Go watch it.