The download has changed to version 0,6!
It is still a mess:
It installed the freeimage.dll onto the root of C:/
This version launches a CLI window with optiPng on every user's change in the UI (for PNG).
(monitored with ProcMon) It launches pngout.exe which launches Optipng.exe which launches a CLI window which then cannot find pngout.exe in the Irfanview/plugins directory, it searches only %path%. Because (in my case) it's 32-bit IrfanView and all plugins on 64-bit Windows, it incorrectly searches in /system32 (yes, I use 64-bit ProcMon and it doesn't look like it searches in /Syswow64, unless ProcMon lies, by why would it...). The only usable and safe place to put pngout.exe is in %Windir% (or maybe somewhere else in the path, if one defined such a custom place for commands).
The problem with Riot-external-tools.ini still stands (cannot delete it, cannot fill it with even one CR/LF, it will be overwritten) - useless zero-byte file when not in use, and sometimes, if it cannot find it, Riot crashes. If it manages to survive, it will recreate it with zero length.
By the way, you can still move Riot.ini and Riot-external-tools.ini to IrfanView root directory (where the IrfanView.exe is) and get rid of the littering in %userappdata%/local.
Also, the Help menu works if you move riot.chm to the same place (where the IrfanView.exe is).