If Aces High is in the default installation path, then the squads folder is located at
C:\Program Files\HTC\Aces High\squads
Put your squad art there and use the "Upload Noseart" button in the "Club" section of the clipboard, in the game. If you do not get a message, then the art has been successfully uploaded.
Once a week I take the art and go through the approval process. Usually on Monday, but this week, I will have to do it tomorrow (Wednesday).