HeLLcAt,
Copy all the bmp files into your \Aces High\Sights\ folder.
In the game use the clip board to go to setup.
Click on the Sights button.
Set each gunsight to the desired aircraft by sellecting the version without the "_alpha" in the name.
For example, I have the P-51D set to k14.bmp. Aces High uses K14_alpha.bmp as a brightness map, if you don't have it in the Sights folder there will just be an orange square in place of the gunsight.