Above was almost correct, except the capitals are not required..
Bit more info (Most logos have transparency rather than a white or black background, for this the pixels you wish to be transparent must be 000000)
The fleetmark.bmp must be saved in the root Navyfield folder, usually (c:/programfiles/sdenternet/navyfield) or similar.
Once you have reached all the requirements listed in the posts above, right click radioman, click on clan, and you should see "register seal" just below the "fleet bank" click on this, and confirm. You will then need to reconnect to see the logo.