If you've got a robot that's been trained to capture a screenshot, and you'd like to see what has changed since the previous screenshot, here are the steps:
Click on the robot from your dashboard
Click on the History tab for your robot
Find the appropriate task from the list
Check your screenshot to see if it says "Changed"
Click on your image to view a larger version
Use the slider to see the previous screenshot compared to this most recent
Note: There is a Highlight Changes toggle that you can use in order to get more visibility into what the differences are between the two versions.