Any chance the Xscreensaver package is installed?
If so, the decayscreen one, set with a gigantic delay like so or bigger even, appears to do what you want if you set the Advanced-->Grab_Desktop_Images checkbox. Kinda hacky, but until you find a real version
maybe it will work for you. There are others that will probably work the same way.
decayscreen -root -delay 1000000000000 is the command-line of the screensaver itself, and `
$xscreensaver-command -lock` will lock the desktop and put up a frozen+locked image of the desktop.
Sasha