Yes, it can. Although you will likely get better results using PNG, JPG or if you're looking for an animation, Flash.

-Eben

Try box.net

Taltos...

Create an image the size and shape you want, and export it to .jpg format, and upload as your banner using the guildportal control panel.

Ideally you'd want a copy of Photoshop, or Paintshop or something of that nature.  If you don't know how, or unwilling to torrent that, you'll have to invest some money in the software.

If you want something free and functional, google The Gimp for windows.  It's photoshop-esque, and you can make some pretty sophisticated stuff if you have the patience to learn how to use it.

Powerpoint is not an image solution because it can't (I believe) export to jpg format.  Though I suppose you could take a screenshot of something in powerpoint and crop it...kind of a cumbersome way to go about it though.

Otherwise you can probably whip up something simple in paint.  Something as basic as an ingame screenshot, cropped to be wide and thin, and text applied can look better than some of the animated monstrosities you see on some of the pages around here.

To make pictures appear that are linked from other sites, you'll have to add an html tag that gives guildportal a way to find the image.  Most of my pictures on my site are pulled from photobucket that way.  The easiest way to go about this is to click on the very upper right hand button on the text formatting frame (visible whenever you're editing content) called ImagePathInserter.  It looks like a couple mountains with a sun behind it.  Click that, and type in the http:// link to the image you want to insert.  You're done!