There are probably many sites out there with decent art. The problem is that unfortunately many people don't bother with "all that copyright business." Which means they're breaking the law (with or without knowing it) by having it on their site to begin with, and that everybody who takes and uses them without permission will also be breaking the law.
There is a copyright myth that 50 or 60 years after a work is made, it's "anything goes" and public. Well, it'd be awfully nice if it worked that way, but that would be disregarding that sometimes the rights to a work pass on to someone else after the author's death... which means that somebody has the rights to it, which means according to law, we haven't.
Now we can all (and I'm sure we've all done this) pound our fists on the table and rant about how idiotic it is that we can't use something that's over a hundred years old however we'd like... but there's this little line in the Bible "Render to Caesar the things that are Caesar's, and to God the things that are God's." Or in layman's terms, unless the law compels you to sin, we're unfortunately compelled by obedience to obey it.
What this boils down to, is that there are a lot of people out there breaking the law just because it's convenient, or they think all of these things are "Catholic heritage" and therefore everybody ought to have a right to see them... but the law stands as it is. I've even come across a family run company that SELLS this art, who, when I asked if they had the rights to sell the images they were selling, didn't respond.
On a lot of levels, these legal technicalities are a royal pain in the behind. Nevertheless, we're bound to follow them. The problem here then becomes, that since so many people defy these laws openly and blatantly, which sites have images that are actually, truly legally free for public use, and which are simply breaking the laws because they don't happen to agree with them.
I can't think of a single site that I could recommend, and say with 100% certainty that they actually are giving away legally free images. Because of that, my own religious art folder on my PC is empty, save for the pictures I have done myself.
It's sad, but... something to consider before going out and getting all these images. Of course being an artist (of words) and not financially well off, I can appreciate the point of view of the rights holders, too.