This is saving me hours and hours on overlay drawing and icon tweaking so I hope that others find this helpful too.
Direct Link: youtube.com/WZdUL5jt6Hk
Summary: Rename the icon to be .png, Download TweakPNG, open the icon in TweakPNG, export the zTXt (second) chunk from the file, make your modifications in photoshop/other, open the file with TweakPNG again and import the same chunk directly under the IHDR chunk. And presto, a DMI file which you edited in photoshop. The video walks you through a few handy things for overlay making too.
Cheers!
The only thing i could honestly suggest is making an add-on tutorial to help people set up a work station alike the one you have made. Showing them how to make separate layers/groups of layers to quickly navigate things. It might also be nice for you to supply what version of photoshop you use.