Open "DATA\INTERFACE\NEURONET\NAVMAP\NEWNAVMAP\nav_prettymap.3db" in UTF Edit, under the texture_library you'll find one texture "fancymap.tga" import your TGA texture into the MIP0 node, if you using DDS then change the MIP0 to MIPS, try and keep your texture to the same size as DA's, as if you make it to big it will get squished to fit and will look pixilated.
I can't remember which one the navmap background is in as it's been a while since I last worked with the HUD. You will need to make sure that your texture is capable of being seamlessly tiled, as iirc the zoom function applies the texture four times in the NAV window, if you don't get the texture right the seams will stand out.
**shuffles off with a new headache**