When submitting a Bug Report, it is very helpful if you attach images that show us the issue. Similarly, when submitting an Abuse report, it is required that you include evidence of spam/offensive behavior in the report in the form of a screenshot.
If you are taking a picture of the website, try to include the URL (web address) in the image if possible.
Windows Desktop
The snipping tool is the simplest solution; read about it by clicking here.
Alternatively, you can use the print screen tool:
Click the window you want to capture. Press Alt+Print Screen by holding down the Alt key and then pressing the Print Screen key. The Print Screen key is near the upper-right corner of your keyboard. (Depending on the type of keyboard you have, the exact key names on your keyboard may vary slightly.) It will save the image to your clipboard, and you can then paste it on any image viewing application (like the ones you use to open photos on your computer) and save it.
Mac Desktop
Press Command (⌘)+Shift+3. Image is automatically saved to desktop.
Updated information and further instructions:
https://support.apple.com/en-us/HT201361
Android Device
If you're running Android 9.0 and above, press and hold the Power + Volume down buttons at the same time for a few seconds. Your phone will take a screenshot. It should be automatically saved in your Gallery.
Updated information and further instructions:
https://support.google.com/android/?hl=en (search for your device)
iOS device (iPhone/iPad etc):
Press the Side Button and the Volume Up button at the same time. The screen flashes, and if you have sound enabled you will hear a camera shutter sound. The picture is saved with the rest of your images in the Photos App on your device.
Updated information and further instructions:
https://support.apple.com/en-us/HT200289
Windows phone:
On Windows Phone 8.1, just press the Power and Volume Up buttons at the same time to take a picture of your screen. These are automatically saved in the Screenshots album in the Photos Hub.