changing inventory backdrop

[ICantAim]

I've seen various images/gifs of people with custom backdrops/images in their inventory screen, want to make my own but can't for the life of me figure out where the image would be located in the files.
I have hafenrestool installed, tried brute forcing my way through by just viewing every res file but I've done it 2 or 3 times now but still can't find where its located.
Pretty sure most of the comments would be along the lines of "if you cant find it, maybe you shouldn't try messing with it" but I figured giving the forum a shot couldn't hurt.

example img of backdrop I want to change

[Nightdawg]

gfx/hud/invsq

gfx/hud/wnd/lg/bg

[ICantAim]

thank you very much