Isn't Venture is it by chance ?
A picture sometimes looks worse enlarged on a projector than small in a print ! Surely you saw the quality of the picture - the composition etc on the wall and if you have agreed to buy thats what you have done - agreed to buy. If you're not happy in any way - see the manager of the store and sit down with the original shots projected on the wall and tell him why you are not happy if the actual prints are different from the projected image. But if the prints are the same - thats that - they have done as you have asked. Have a look at the copy of the order you gave them when you ordered the prints.
We've dealt with Venture twice - had no complaints - yes they are expensive - but I think a little unique and worth it.