This feature is only available in the desktop editor.
If you would like to automatically scale all images that you add in the desktop editor you can specify the maximum width and/or height in preferences. The original image data will be saved and you can restore the image to full size later on if you would like.
When you scale images in the desktop editor you are setting the maximum width and height for the image when it is uploaded to the server and displayed to the user.
Refer to our article on How to open Preferences in the ScreenSteps desktop software