Lets you choose different protocol icons for different accounts so that you can tell the difference between, say, a personal XMPP account and one that you use for business.
After installing and enabling the plugin, go into your account preferences into the advanced tab. Down the bottom should be a new option 'Protocol Icon'. Fill that in with the name of an image file in your pixmaps/pidgin/protocols/16 directory, excluding the '.png'. eg if you want to set the Yahoo icon, type in 'yahoo'