PAINTdotNET is another easy transparent making program, juz remember.... GIMP is almost all te way similar to photoshop....

Gimp>Layers>New>Transparent
(I think this is right, haven't used Gimp in a while)
Then you can rub out and stuff.

Png is the best format for textures as they don't compress and lose quality and it supports transparency.

Open GIMP, do 128 256 or 512, click advanced options and there should be a tab you can click on. Scroll down to transparent and make whatever texture you like. Save it as ".tga" and then a screen should come up with a box tic mark in it. UNCLICK THE BOX. After that just upload it to your tori and give it a few minutes to show up.