This was first posted by Noobcraft over at MMOwned and I take absolutely no credit for it but since it is not here I thought I would fix it up and post.
1. Go to WoWHead and look for the item that you want ie. Arcanite Reaper.
2. Add &xml to the end of the page ie.
Arcanite Reaper - Item - World of Warcraft becomes
www.wowhead.com/?item=12784&xml.
3. Look for something like <icon displayId="23904">INV_Axe_09</icon> on the page. By the way, it will be different because you have chosen a different item but the first part should be the same.
4. When creating an item put 23904 as the Display ID.
That is all you have to do. Have fun! =)