Easiest way I have done this is to drag a copy of the app I want to replace into photoshop, overlay the image on the original and edit to my specifications and save. Rename the original and then drag my edited version back in to the folder. I believe Gimp will do this as well if you do not have photoshop.
Also recommend that you keep Gridlock because it gives you the freedom to place your icons wherever you want if you are/are not using widgets. Gives you a personal feeling to be able to leave blanks slots in places and it does not affect performance at all.
Editing icons sounds a little difficult lol I was just using the glasklart icons, but without the mask, just the white image portion. They are smaller than regular icons though.
I have read on a couple different forums that people who use gridlock have experienced quite a bit of battery drain. You don't think it eats up your battery? I could never tell what was killing my battery life because I always had so many tweaks installed at the same time. Currently I don't need it, but I'm not opposed to reinstalling it if I have a wallpaper I want to use and it would benefit me to put the icons in different places.