I just installed CM12 on my Galaxy S3, and was trying to figure out how to take a screenshot. There are two way you can take a screenshot in CM12. The first option is by holding the power and volume-down buttons at the same time. And the other option is by enable the ‘Screenshot‘ in ‘Power menu‘.
Step-by-step for option 2:
- Enter Settings and press Buttons under Device.
- Press Power menu under Power button.
- In Power menu, click to check Screenshot.
Now when you press and hold the power button you will have the screenshot option in the menu like the image below.