One option that's good if you're careful with picture choice, particularly if you'd like to try a few things out before shelling out for a massive job from a proper printers (particularly if you have access to a printer at work

) is to rasterbate an image. The advantage is even relatively low resolution pictures can be blown up massively and look great and you only need access to a regular printer.
I had some pasted on the back of a temporary garage door and thought it was pretty good. Most people tend to have gaps and tile the image, but I have no idea why since you can line them up to get a seamless picture. That could work out well though if you're putting the image behind a window frame that has panes. Works better the further away you are, but then that'd be try for any attempt to create a 'fake' room behind a window.
It't at least allow you to check the image. The one posted earlier with the Mustangs for example may look weird and not work since it's taken from a top down angle. On the other hand it may work and just look as though your shop is on a higher level, who knows until it's on the wall. But even Kinkos style 'cheap' blow ups aren't going to be cheap cheap for experimentation.