OS Windows
Open in a browser the page you want to get an image of.
Press the PrintScreen (PrtSc) key on the keyboard. This button is most often located in the upper right corner of the keyboard in the same row with the F1-F12 keys.
Open one of the graphic editors: for example, the standard Paint program.
Press Ctrl + V: the image will appear in the editor.
Complete the image with the necessary elements: inscriptions, lines, etc.
Save the resulting image: "File" → "Save As".
Mac OS
Navigate to Applications / Utilities.
Launch Screenshot (Grab.app).
Go to Snapshot → Window.
Circle the required area on the screen using the appeared cursor;
The resulting image will be automatically saved on the "Desktop".