Go to Advanced -> Gameplay Interface (or sumthing like tht), and search for the icons (that might take a while, I don't know the exact name off the top of my head so I can't give it to you). Then, to change them, check the "Use Custom Values" box and change the icons.
You will probably need custom-made icons if you are going to change those particular icons. I'm pretty sure wc3 autoresizes icons to fit into whatever place they're supposed to go in, but sometimes it still looks ugly.