Ok, Some interesting Question for me. A bit of reverse engg. 
Here we go:
1. Get a Resource Editor. I strongly recomend Resource hacker 3.4.0.
2. Open the yahoo messenger (or any other) Executable in it.
3. You can see the resources in the exe there. In the Left Pane Choose Icons> Icon No>Right Click> Replace.
4. Browse any other Executable or icon file.
5. Repeat Steps 3 & 4 for all other icons remaining.
5. Save exe & Enjoy!
Download Resource Hacker 3.4.0
http://www.angusj.com/resourcehacker/
Here we go:
1. Get a Resource Editor. I strongly recomend Resource hacker 3.4.0.
2. Open the yahoo messenger (or any other) Executable in it.
3. You can see the resources in the exe there. In the Left Pane Choose Icons> Icon No>Right Click> Replace.

4. Browse any other Executable or icon file.
5. Repeat Steps 3 & 4 for all other icons remaining.
5. Save exe & Enjoy!
Download Resource Hacker 3.4.0
http://www.angusj.com/resourcehacker/
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