Did you goto the link posted above? Did you read how to pull the "DEFAULT" (which is the only one you can edit) skin out? Once you went into the game and saved the defaualt skin that you want to work on did you ....
Go to the Aces High II directory. This is usually in your program files directory. Under HTC\Aces High II\skins you will now have a new directory. This will be the name of the plane you chose. For example, in the image above I chose the 109K-4. The directory I need to look for is named "109k4" inside this "skins" directory. It could be p47d25 or p40e or b17g or whatever you are looking for. It should be self evident.
When you open the folder with the name of the plane you want do, you will see all of these .bmp files, those are the ones you use.
You CAN NOT open any skin you want and edit it. So spit8_7 (which is the 7th skin for the spit8) can NOT be opened by the art program.
Print out the directions on Krusty's page in the link above, and follow them step by step. It will get you started.