I have a minimalist installation of Ubuntu 13.04 using Lubuntu minimal package. By default it does not come with screen capture utility. To get Print Screen function working in Lubuntu, install scrot, like:
Pressing PrtSc in Lubuntu will cause it to create a file in the home directory normally named <date>_<screen resolution>_scrot.png.
In Ubuntu 13.10, you have to run scrot first before PrintScreen works.