Hello mou4ex thank you for posting such an interesting mechanic and welcome to the Unreal community.
I tested the game and it seems to use the desktop folder for its image gathering.
So out of the box, with the engine, I was not able to achieve the goal you are going after. (If anyone else knows how to achieve this, I would love to see what you would do differently.) I took a look at what the game is looking for by using Process Monitor by Microsoft.
It seems the game is checking my registry to get my desktop folder (which on my own PC, I moved to a non standard location). From what I gathered, You may have to do this in C++. You will likely have to make the calls platform-specific as Mac may have a different location/nomenclature for its folders.
Here is a relevant thread on querying a registry.
I went into my Registry Editor and found that this key will expose my current location of my pictures, documents, etc. “HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows\CurrentVersion\Explorer\User Shell Folders”.
This is a generic path and is Windows specific.
I hope this helps.