Bioware's official Neverwinter Nights site includes a selection of wallpapers.
An interesting download you might want to play around with is the Model Viewer. You can download the model viewer plus a selection of in-game models you can examine. With each model, you can rotate the camera and watch various animations programmed for each model.
Neverwinter Vault is perhaps the place to go for information and downloads of all kinds. The site has switched over to focus on Neverwinter Nights 2 now, but if you scroll down far enough, the menu bar on the left will show Neverwinter Nights links and resources.
Helping you sort through the mass of resources out there is their Hall of Fame, where downloads that have received excellent votes are showcased. They didn't have a proper rating system in place and there was a lot of bad and meaningless voting, but in general, if a download has made it here, it's a pretty safe bet that it's really good.
Their section on screenshots, sometimes from modules and often with camera angle restrictions lifted due to scripting (programming) in a module, is definitely worth a look if you are keen on fantasy art.
There is also a small collection of concept art for the game if you like a "behind the scenes" look at the artistic process.
The section on portraits has several pages of fantasy portraits that's worth a look as well. Few are likely to be completely original works, however, and are more likely deviations or wholly lifted from another source for personal use in the game.