It's a simple process to make any kind of semi-transparent texture for Second Life, if you are starting with an image that has dark areas where you want the transparency, and light ones where you don't.

All you have to do is copy the image, paste it as a new Alpha channel, and adjust the levels to make sure that the black areas are 100% black, which will show as fully transparent, and the white areas are 0% black, so that they'll be completely opaque.

If that sounds like Geek to you, just watch the movie. All will become clear. (Well, the parts you want to be clear, anyway.)

Enjoy!