MasGrafx is one of the leading NR2003 sites, as well as 'The Pits.'
For NR2003 painting; start with a PSD or PSP template. Make your paintjob (I recommend using the carviewer, available on the pits website, to view your progress. Pretty much a less-glamorous CMX Viewer) using the template. Once done, save it as a TGA file and place in the "Import/Export" folder. (specific format escapes memory, just try different options :P). In the game, create a new car in the "opponent manager" screen, being sure to select the proper chassis that the template requires. Some of the newer templates require the chassis of a different car to look right. For instance, a 2006 Dodge Charger template I think belongs on the Pontiac chassis because it has a nose shape closer to the charger. Then, after the car is created and you are viewing the newly created car in the opponent manager, click on the 'import' button, and select your paint job. Use the same method for your pitcrew, only make sure you have the pitcrew tab clicked in the opponent manager screen before importing the tga file.
Do
not use the in-game painter; not even for importing your paints. In mods it will mess up the paintjob, and in the standard cup cars it will make the paintjob smaller (512x512, ugly!).
MasGrafx has a wide range of purdy paintjobs available, too, as well as some tracks. I recommend getting the Project Wildfire mods, too. The Craftsman trucks are particularily great.
Hope that helps