If you're not using KDE, I don't know.
If you are using KDE, then you can use the KSnapshot program.
It should be in your K menu under Graphics, but you can also use Alt-F2 and then type ksnapshot.
If you want to use the keyboard in KDE, the keyboard shortcuts are configurable. I think the defaults are:
Alt-PrntScrn for a screenshot of the current window
Ctrl-PrntScrn for a screenshot of the current screen
You can see what these are on your computer by using the Control Center, and going to Regional & Accessibility, then Keyboard Shortcuts. Look under System, Miscellaneous, and you will see what they are. Feel free to change them if you want.