GConvert 5 edit icons

Hi, I am using Windows 8.1 and installed GConvert 5 over Christmas because I was looking for a way to give my machine a fresh more unique look. The main places where I view icons are on my Start Menu, on my Desktop and in Windows File Explorer.
I read that the built-in icons are stored in a dll library folder c:\windows\system32\imarages.dll
On opening GConvert I have clicked on Tools and then Modify Icon/DLL… I then navigated to c:\windows\system32\imarages.dll
Many of the icons that I would like to find alternatives for were shown and I tried clicking on a few of them to experiment with the program.
For example, I double clicked on icon 108(1033) which is the Music folder icon. I then navigated to an alternative icon that I had downloaded to my desktop, I then get an error message saying "Cannot start resource session."
Either I have misunderstood your program and replacing the Windows built-in icons is not possible, or I am doing something silly, please advide

I believe that the DLL you mentioned belongs to the Windows’s protected system files. Such files are protected by Windows against modification, so you can’t modify them with GConvert. GConvert lets you edit icons in DLL and EXE files, but with restrictions.