This should be fairly easy to do yourself.
- Download the icon from Wowhead
- Open it in an image editing program like GIMP (not MS Paint)
- Edit the image until you're happy with how it looks
- Save it as INV_Garrison_Hearthstone.tga
- Create a folder called ICONS in your WoW\Interface folder
- Put the edited image in the ICONS folder
- Restart WoW
Your custom icon should now be displayed instead of the default one.
If it displays as a green square instead, try follow
this guide to save the TGA in the proper format.