Set W3M Image viewer

W3M defaultly uses FBI (in my case) to view images. FBI uses frame butter as Zhcon. This causes problems. When Zhcon is running, fbi will not be able to be used. Use zgv will get rid of this problem. To tell w3m to use zgv, tow steps:

made a .mailcap file in the home directory and add this line:
image/*; zgv %s

add this line to the ~/.w3m/config file
mailcap ~/.mailcap