Well, this is at least the wheel that looks like the one on the attackmap screen shot:
http://www.subsim.com/radioroom/down...o=file&id=1601
But it is also intended to replace your uboat icon. I don't know which file is responsible for showing a target on the nav map, or even on the attack map if that uses a different file (I doubt it). I suggest you investigate yourself which file name it should be, by using a image viewing application that can open .tga files. I personally prefer XnView for that. It's probably just a mater of replacing the file.
But make sure you keep a copy of the original!!!!!!