I wanted to take screenshot within Ubuntu guest and paste the screenshot into an image editor.
PrtScr or Alt + PrtScr doesn't seems to work.
Ubuntu comes with a Screenshot tools.
I usually set Grab the current window with Grab delay of 3 seconds. Click Take Screenshot and select my target window.
It offers the option to copy screenshot to clipboard or save to file.
NOTE: Some recommened Shutter (sudo apt-get install shutter) but I have yet to try it.
NOTE: You can take screenshot of the guest from the host using Host (Right Ctrl) + E key. I will prompt the save the screenshot file on the host OS.