Yes, but keep it to about 20K or less so that page loading and content aren’t impacted too much.
There are two basic options, if your webserver supports php you can use anole’s php sticker script described here, or you can have WD create a custom screen and upload that to your webserver, there’s a FAQ for that. Either way put the URL into your profile, for instance mine is a php sticker and I have:
[img]http://www.realweatherstation.com/wxs/sticker.php?type=banner[/img]
in my signature box.