There are several ways. I will cover two. First way, using e17-supplied tools:
Left-click desktop, go to Settings > Modules. In the left pane, under the Shelf section, find and click on Screenshot module. Then click the big Load Module button at the bottom. Close the window. You can now add it to a shelf by right-clicking the shelf and selecting Set Shelf Contents. In the left pane, find and click on Screenshot module, then click Add Gadget button. To add the module to the desktop, left-click on the desktop, then select Settings > Gadgets. In the Gadgets Manager window, find Screenshot, click it, then click the Add Gadget button. It may end up in unpredictable places on your desktop. No problem. Right-click the Screenshot gadget on the desktop, select Begin move/resize this gadget, then drag it where you want it, and resize it if you want. To make it "stick", right-click on the Screenshot gadget again.
Before installing the 2010 e17 version, I was running e17 on top of PCLOS 2009.3 LXDE edition. I installed ksnapshot from the repos. If I remember correctly, there was only one dependency. It didn't want to bring the KDE base with it.
EDIT: The ksnapshot package isn't available separately in the KDE4 repository. So that method won't work any more.