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Graphics

Instant messaging users are generally very particular about the appearance of their IM client, and with good reason.  If you are going to have a client open all the time that you are using your computer, it can easily grow tiresome.  Remember that your client is probably at least partly obscuring the users favourite backdrop picture

There are some good resources out there for creating icons and graphics.  Microsoft has released a tutorial on how to design icons for WindowsXP, which includes new icon sizes.  It may be worth reading this if you want to design a client that doesn't look out of place on the next generation of Windows desktops.

Thanks to Blancmange for the original LED style icons on this site.

Skins

Skins are immensely popular, as can be seen by the number of sites dedicated to skins for apps such as ICQPlus and WinAmp.  It is the same need to personalise what is ours that causes people to change the cover on their Nokia phones and Palmpilots.  

Skin support can take the burden of designing the perfect graphics off you, however if attention has been made to presentation, I would not regard skin support essential for a version 1.0 release of a Jabber client.  Options to let the user easily change the colour, fonts and status icons go along way toward enabling personalisation without having to support full skins.

Branding