Hi
This is not hard but timeconsuming. The original game hangars are in:
\Games\World_of_Tanks\res\packages.
The hangar .pkg file has to be unzipped (unrared). You will get multiple
folders and usually content/interface/shells/... contains the item you want.
Then you have to open the .dds file where the item is with Photoshop
and do all the changes. It can be for example: hangar_p_details.dds.
Then save, zip back into original .pkg and save.
If its a mod hangar then everything is in res_mods.