Okay, I am going to show you how to make clear textures, or partially clear textures, for body parts and trails, using GIMP 2.

First, open GIMP.

Second, open a new image.
Make width and height each 512 (it doesn't matter what size texture you have, this works best).
Next, click advanced options, change X res. to 128.005, and Y res. to 256.005.
Almost done, now change the "fill with" to transparency.
Finally, when you are done save as .gif or .png.



*If you want it completely transparent, then just save the image after your do all of that^.

** If you are going to insert an image into the transparency, what ever looks like a checkerboard will be transparent.