Every folder courtX corresponds to a court. For a 2D you need a court.jpg for the texture and a court.ini for the physics. To replace one of the default courts overwrite a court.jpg in of the original 9 folders.
For adding a new one you need a new folder called CustomCourtx (Starting with 1, not 10) in which you should put the court.ini and court.jpg (and the files for the Umpire and Animated Line-Judges if you want them).
About 10 custom courts will appear in the training club, the rest only in the tour. To make them appear on tour you need to edit the tour.ini. Near the top there is a list of courts (under [court]) add your new court as 10 (for example (MI = 10) it doesn't matter what name you give it. To assign the new court to a tournament, find it then replace the entry in the "Ground" field with the name of the new court.
I think you getting confused with stadiums and courts. A 3D court has both a court texture (courttex.jpg) and a stadium texture (Courta01.jpg for outdoor, Courtin1 for Indoor). courttex is found in the court folders in data, courta01 in /models/textures/stadium/subfolderx. To add new stadium textures you need to create a new subfolder in the Stadium folder (can be called anything) and put a Courta01/Courtin1 file in it). To assign it to a court you need to edit gamesys.ini, adding new entries under [stadium] at the bottom of the file. The "skin" value corresponds to the folder name of the texture. There are several other options you may need to set as well, for indoor stadiums you must add the line Mesh = StadiumIn01.
To disable the default advert textures (most custom courts don't use them) add the line SignColor = 0.
Hopefully this makes sense, I know this can seem kind complicated and confusing, but it shouldn't be too hard to understand