Brightness control for backgrounds

There are several backgrounds that we'd like to use from the catalog, but they "pop" too much for our sanctuary and church culture. We're going for a subtle look. Is there any way to make the backgrounds dimmer so that they appear more muted?

I've considered using photo editing software to adjust them, but I think that might violate the image copyright terms. Plus, it would be a hassle to edit each one :-).