Behind the picture is a pressure plate that you can throw an item at to trigger it.

Instructions:

You don't use glass, of course, but a block that you can't see through.
To place the image, you must first place a normal block instead of the pressure plate. After that you can exchange it with the pressure plate.
Unfortunately there is a small visible gap next to the image. To avoid this, you can either use a wider image or place other images or blocks next to it.

The switch can then be connected to the classic door behind another picture, for example.