Let's Say I want to see this screen with the bouncing ball...
1) Figure out what you want to unlock the next screen. In this example I want the ball on screen 2 to show when the button on screen 1 is pressed.
2) In the graph behind the screen you want locked, create a variable. Use conditionals to set it to 1 when the conditions are met and 0 when they are not (see image below).
2) Upload the image you want to block the screen. In the coordinate for the center put the restriction to have it only show when the condition for unlocking has not been met. That way the lock screen will disappear when the condition has been met. You may also want to find a way to keep the image from being moveable. I like to use a list but that's up to you.