Second step should be easy, open up the unit editor and find the items folder. Look for the 'interface icon' field for any item. Replace this icon with any icon you want.
Step 1 is a bit tougher. You need to extract a bunch of files from war3.mpq to find the icons for abilities.
Get WinMPQ or mpqView to open up war3.mpq (in your \war3 folder). Google for these progs to find them.
Now when you have it open. look for any file that is of the following format:
**AbilityFunc.txt
where ** is any of the 4 races in war3 (human, orc etc.)
If you look inside these files, you will see the paths for the abilities written inside them (blp files). Just copy and paste the one you want into the interface icon field for an item.
Ghan has said he has fixed this. Monovertex please confirm this fix. This was only a problem with people that had signatures in the upper levels like not the special members but the respected members.