You need to make a slide out of the image and blast the spotlight intensity way up.
Here’s a blend file with an example. I worked up a simple projector.
Everything is parented so you only have to aim the projector.
To change the picture go to the ‘slide’ material of the projector and change the image file.
-To make a brighter picture (for daylight scenes), up the spotlight brightness. (can’t do this in the real world!)
-To defocus the picture change the spotlight size. Perfect focus requires size = 0.00
-To change the projection size move the control on the box.
-To simulate a cheaper projector change the spotlight blend amount and/or angle.
This was kind of fun, and a keeper for my library!
edit: I just realized that one limitation of the model is that the soft shadows cast by objects in front of the screen is tied to the projector focus. A more realistic projector would generate sharp object shadows even if the picture focus is set very blurry. I’ll think about this. Using two spotlights comes to mind…