To begin you'll need WinMip2 & an editing program (Paint Shop Pro, Photoshop, maybe GIMP).
You use WinMip2 to unpack/explore a particular track's .dat file. This is usually where everything is kept for that particular track.
For example, let's use the default Homestead track as an example...
Launch WinMip2. Be sure the little NR2003-looking tire is selected.
Click File > Open and navigate to the homestead track folder (assuming a default install: C:\Papyrus\Nascar Racing 2003 Series\tracks\homestead).
Double-click the homestead.dat file.
You'll notice a file called 'billboards.mip'. Double-click it.
To the right of the billboard images you'll see the settings for the .mip file. Write these down...you'll need them later.
Click the little 'down' arrow next to 'mip' and select 'bmp'.
Click on 'File > Save As' and then 'Save'. This will save the .bmp in the root homestead folder. If you wish to save the .bmp someplace else then you'll need to navigate to where you wish to save it 1st (or move it after you've saved it in the homestead folder).
Using your design program of choice modify the .bmp to your suiting. Keep the aspect ratios of the billboards you're changing the same as they are.
Save the .bmp with the same file name.
Back in WinMip2 select File > Open and navigate to where you saved the .bmp and double-click it.
Click the little 'down' arrow and select 'mip'. Remember those settings you recorded earlier? Now is the time to use them to assign the .mip settings.
Save the modified billboards.mip to the root homestead folder. There is no need to unpack and repack the homestead.dat file as anything outside of it will overwrite what is in the .dat file.
I think that's everything. If all went well your new billboards should now show up at Homestead.
Now just repeat all of these steps for every track you want to change the billboards at. As far as I know there is no 'master' set of billboards common to every track (probably because the billboards aren't the same size from track to track ~ that's up to the discretion of the track maker) so this has to be done on a track-by-track basis.
Have fun.
