When you create a new action (or edit an existing one) press the "more fields" button. Then select "Pictures & files" and press the camera button.
This will open the camera so you can take a picture or your check.
Don't forget to press the "confirm" button to save your changes.
tip-
If you use a lot this feature you may want to add a shortcut for it on the "Actions" screen.
To do so long press the line of buttons at the bottom of the "Actions" screen and select to add a new button. Then select the camera. This will add a small button with a camera to the "Actions" screen. Now, whenever you press this button it will create a new action + open the camera in one click!