The good news is that Ubuntu 18.04 comes with its own take screen shot features.
If you need to take a full screen screen shot, just press the prntscrn key on the keyboard. The screenshot will be saved in the default location which is the /home/yourusername/pictures location.
Now if you need to take screen shot of a particular section then please use the “screenshot” app. Which will be available in the search bar.
By using this app you can take screen shot of your selected area or your current window on which you are working. You can change the patch of the screen shot by changing “save in folder” location.
Enjoy Ubuntu!!!!