Most of this is covered in the skins forum. This proceedure replaces the default skin in your terrain.
Before you begin, make sure your terrain will build without error.
Open AH2 in offline mode.
Select the RV8 on the clipboard in the hanger.
Right click the RV8.
On the dropdown menu, select Save Default Skin.
Exit the game.
Find C:\Program Files\HTC\Aces High II\skins\rv8\RV81.bmp
Make a copy for editing
Open the copy with your favorite graphic program.
Make some random changes you’ll be able to see in the game.
Save the file, 8 bit (256 color), don’t change the size, in this case 512x512, 257KB.
Rename the file RV81.bmp if it isn’t already.
Move the RV81.bmp file into your texsrc folder.
Open the TE.
Open your terrain.
If you have problems after this, open another terrain, then reopen your terrain, you may also need to clear the cache file occasionally but I haven't had that problem.
Build your terrain and remember to move the *.res file to the AH II \ahiiterr folder.
Fly your terrain in the RV8.