You might try clicking the "Prt Scr" (Print Screen) button while playing NWN, it should (for some it does not I have read) generate a targa-file .tga (picture format) to your nwn's root directory (default I believe would be C :Program Files\\Black Isle\\Neverwinter Nights\\ (or something similar)). This file is rather big depending on the resolution you are using while gaming, could be several megabytes. But it's accurate and lossless as far as I know. You can open it with programs like MS Paint or Irfanview and select "File -> Save As" and from there select the file format for the new copy to be .jpeg / .jpg. This will make the image much smaller in disk space usage, like several hundred kilobytes. It will render the filage slightly less accurate, but with the default settings the difference is not big when compared, unless you are an artist of course.

Hope it helps.