a small ceremony & a big screen! How can we decorate this?

I would try draping fairy lights down all the way across? That might look really pretty.

Hang clear Christmas lights from the top. Or put a floral/greenery garland on top.

Is this a venue that has had other weddings? Maybe you could ask the owner what other couples have done in the past, or search pictures of the venue to see examples. My idea would be to place a pipe and drape in front. I think that if you put up a black image it may emit some brightness which could be distracting. I think that a screensaver type of thing might also be too busy. Also, is it possible to move the curtains in front if it?

I would discourage you from having an image or lighting the screen. It might interfere with some of your photos and also be distracting. Pipe and drape could work for sure, especially if you had a second fabric draped over the top and down the sides to cover the pipe and visually frame it.