How to change [U4] U-Boat Combat Mat image in The Hunters: German U-Boats at War, 1939-43 module?
If you have 7-Zip, right click on the module and you should have a number of options to open the module file or extract files from the module. It may work with other than 7-zip too. If you extract the files, all the images including the mats will be in a folder called “images” so you can look for the mat you want to replace with it’s file name. Make sure the file you are replacing has the exact same file name and is the same type of file (eg. jpeg) before you drag and drop it into the module using 7-zip. Also, create a copy of the module before doing this as sometimes it may not work. If it doesn’t work then you will have to use the editor to add the Mat which is a little more complicated. Well that is what I do anyway someone else probably has a better way. 
Cheers D.
I forgot to mention that if you change the vmod file extension to .zip, windows should open it as well. The module is essentially a zip file from what I understand.