Using sacred art in catechesis is an excellent way to draw youth and adults into the beauty of our Catholic faith. Care should be taken, however, to respect artists and the copyright of their works.
Wikipedia Commons (http://commons.wikimedia.org) contains a large selection of public domain sacred art work; just search for a particular Bible story or saint.
In addition, the following sites offer copyright-free photos and/or videos that you can use in catechetical presentations: