Just open up the .png in an image editor, the text is stored in the nonimagedata file, which you can use notepad/notepad++ to edit. I find that the res_0 layer is the skill name, whereas res_1.nonimagedata is the actual skill description. The skills themselves as in the character sheet are located in *\res\gfx\hud\skills
As other people said, remember to use theTravs tool to decompile/encode it again. Also, you had the same idea as me, I'm guessing (Actually completing skill descriptions would help for the newbies).