As you research your saint you will find images of them in the art reference books, regular art books, journal articles and in these databases.
Search the name of your saint in the databases (last name, first).
Search the name of your saint in the UNT Libraries catalog to find books that are all about your saint or have a chapter on your saint.
Also be sure to check for your saint in the books listed on the "Iconography of Saints" tab in this guide.
Post-Biblical Saints Art Index: a Locator of Paintings, Sculptures, Mosaics, Icons, Frescoes, Manuscript Illuminations, Sketches, Woodcuts, and Engravings
The Oxford Dictionary of Saints (ebook, click to read. If accessing off campus, you will be asked for your EUID and EUID password)