Hi,
You will find the file here: (adjust for your install folder)
C:\Ubisoft\Silent Hunter 5\data\Textures\TNormal\tex
and the file name of the black cat emblem is: U-1131.dds
You will need a program to edit the dds file, Nvidia do some free here:
http://developer.nvidia.com/legacy-texture-tools
I prefer to use DXTBMP....you can get it here:
http://www.btinternet.com/~mnwright/programs/dxtbmp.htm (right at the bottom of the page)
Hope that helps, the amazing modders on subsim may have a better solution, I am still very much a learner
Com1