Yay! I have successfully imported graphics into the Celtics court graphics files using Console Texture Explorer, OPTPix Image Editor for PS2, and a hex editor! This took quite a bit of trial and error, but as you can see, I've got a custom Sonics center court logo on the court and a custom baseline logo as well!
The big hurdle I still have is trying to figure out how to change the color palette and having it transferred to the MSH graphic file. I changed the palette in the texture (TM2) file and it shows up with the new colors in Console Texture Explorer after editing, but it defaults back to the old color palette in the game, so that's why the colors in the center court image isn't eye-popping yellow and emerald green.