It can be edited through the config.cpp line for picture in the cfgvehicles section:
picture="\addon_name\pic.paa";
BUT, as The real Armstrong mentioned, I dont think the Invasion 1944 guys would appreciate you editing their addons.
And if you do it against their wishes, and release it, you will make a lot of enemies.
If you do want to edit the units, contact the appropriate member of the mod and seek their permission first.
If you are editing it for your own personal use then there shouldn't be a problem.