When using Hexen with its addon Deathkings, I tried putting the path to a zipfile containing "TITLE.png" highres replacement (in hires subdir) into the zdoom.ini file under the section [HexenDK.Autoload]. It didn't load. When putting it as a separate file in ZDL (as external file), it worked just fine. I am sure I made no mistake with file naming or entered path, works just fine for other files in other games, anyway. Tested with latest available GZDoom snapshot, r1077.
File location on hard drive: E:\GZDoom\highres\gui\addons\hexendk_add.zip
Entry in zdoom.ini file: [HexenDK.Autoload] Path=highres/gui/addons/hexendk_add.zip
Structure of hexendk_add.zip: hires\gui\menu\TITLE.png
I have learned that any folder below "hires" doesn't seem to matter at all, you can put the graphics into any folder, game would find them. Works like this for Doom, Heretic, whatever. Repacking the file to make it look like hires\graphics\TITLE.png didn't help. The irritating thing is, as I stated, that the file works when loading it via ZDL, but not from within the ini file. So it apparently can't be a mistake by placing the file into the wrong folder inside the zip.