My band are thinking of using a projector to show a series of images behind us during gigs. Now, I know this can be done, but I have a fear that we are heading for a 'Stonehenge' moment (we want a massive wall of imagery, but we get a tiny A3 show that were in danger of kicking over.

Has anyone tried this before? If so do you have any advice?

I haven't tried it before, but I deal with projectors for my job, so I thought of a few things to consider...

First, how big of a screen are you thinking of using? If you want it to be a backdrop for the whole stage, you're going to need a BIG screen. It will depend on the stage, obviously, but you're going to need something like a 10' x 14' screen at least.

The bigger the screen you get, the brighter the projector you're going to need. If you're using stage lights that will splash up on the screen will complicate that even more. If you're using a 10x14 or so screen, you're going to need something pretty bright, even if you're dealing with a fairly dark room. If you're dealing with a lot of lights, you're going to need something REALLY bright. You can rent the projector, but if you're planning on doing this regularly, it's probably going to be most economical to just buy one. For that 10x14, I wouldn't go with anything less than 2000 lumens, MINIMUM.

Third, where are you going to put the projector? If you have room for it, you can do a rear-projection setup, but that will eat up a LOT of room in the venue. You can set it in front, but then you have it out in the audience, and leaving an expensive, fragile piece of electronic equipment out in a metal show probably isn't a good idea. You can also mount the projector if the venue will allow it.

+1 what bassman1185 said.

Also think about the lens depending on the distance.
If you have a low power projector , you'll have to put it close , so you'll need a wider opening.

If you do front projecting , it is better to rig it.
It will obviously be upside down then , think of all the stuff to rig it that way. ( cheeseborough, pole , safety chain , the correct plate/screw for that projo , and enough cable to feed the signal )

You'll have to focus the lights away from the screen if there is always an image going on.

My band does a Pink Floyd tribute so we use a lot of projected video and images on to a circular screen rigged at centre stage rear - usually above the drummer, but if it's not a very high stage then drummer has to move sideways. So think about stage height ... it can be an issue getting a high enough projection channel. Also look for a projector with a wide zoom capacity. We usualy rig our projector on the lighting bar, but if we're using venue rigging that might be really close to stage front, or it might be well back - you'll need to be able to adapt to either. And if you want really wide images you might be looking at multiple projectors. Good luck!

Also, be ready not to be able to use it. Remember to give yourself enough time to rig it before doors, so that your not a pain in the ass to the venue either. Rear is always best, only available in larger venues. I have seen some really last minute dumbass projector contraptions. Consult a video professional about all the above listed issues.
If done properly it is a good thing. If not...... well.