How to run OxiPNG with launchd and WatchPaths?

I want to run oxipng *.png in /Users/user/Desktop/ immediately after a PNG screenshot (Cmd-Shift-3) is created in my Desktop folder. I have the following plist file in ~/Library/LaunchAgents/ :

Label
    oxipng
    ProgramArguments
    
        oxipng
        *.png
    
    WatchPaths
    
        /Users/user/Desktop/

Then, I run launchctl load ~/Library/LaunchAgents/oxipng.plist. However, the PNG are never modified after creation. ----- How do I make oxipng run after a screenshot is saved to the Desktop folder?