Needs to be 20X20, not smaller, not larger. Named FleetMark format BMP.(So FleetMark.bmp if you can see extentions) in "C:\program files\SD EnterNET\NavyField" folder.
In-Game:
Right click on your radioman, hit on the clan button in the top right corner, make sure you're on the first page, hit "Register Seal" under the "DEPOSIT" button, relogin, if you did everything right, your fleet tag should appear.
Hint: "Black" RGB: 0,0,0 is transparent in-game.