There are some age old optical illusions that are especially effective as computer images because of the bright contrasts displayable on screen. Take a look at some that I've collected and/or re-drawn.
How many black dots can you count? Look carefully!
These horizontal red lines are nice and straight, right?
Move your head towards & away from the screen while staring at the dot.
Would you believe that the squares marked A and B are exactly the same shade?