The Random Image Web Part was released this last week for MOSS 2007. We have also released installation instructions and a quick tutorial with how to set the web part up.
As with other web parts if you are having troubles after installation, modify the web.config entry.
Where it says:
Trust = "WSS_Minimal"
Change this to be
Trust = "WSS_Medium"
That should do the trick.
Documentation is located at:
Random Image Web Part Documentation