Here are a few more:
How many men can you see?
Can you read this?
Who is drawing who?
Look at the black dot, after a while notice the fog surrounding it.
Try looking at the black dots.
Can you see a face and a text?
Stare at the center of the picture, and move your head closer to the screen, then move away from it. Notice anything?
Do you see the letter B or the number 13?
"If guns don't kill people, people kill people - does that mean toasters don't toast toast, toast toast toast?"