Ubuntu 18.04 shortcut keys not saving screenshots
Here's a screenshot taken from the Ubuntu docs using shift+printscreen.
The point is, if you want the screenshot to be saved to ~/Pictures, don't use ctrl.
The key combination Ctrl+PrtScr captures your whole screen and copies it to clipboard, it doesn't save the image anywhere. You can then open an application (e.g. GIMP or LibreOffice Writer) and paste the captured screenshot.
Similarly, Ctrl+Shift+PrtScr lets you capture an area and copy it to clipboard.
And Ctrl+Alt+PrtScr sends screenshot of active window to the clipboard.