In this work I've been very happy to find Wikigallery. I have used this site extensively for the religious images in the CatechismClass.com programs. As an example, look at the beautiful painting above at the top of this post. I was in awe by the beauty of this painting, and through my work, I found already found at hundreds beautiful images of artwork that I have never seen beforehand.
When you have a moment, search the site using Catholic keywords (e.g. Mass, Eucharist, Sacrament, Christ, our Lord, etc) and you will find a plethora of holy artwork that should be more widely spread throughout the Catholic Blogosphere.