well I made my own set of pictures when I was a new catechist, used 5x8 cards or full sheets of card stock, cut out pictures from magazines, calendars, greeting cards, religious goods catalogs, old used textbooks etc. One set is the parts of the Mass, another set is the sacraments, a set on the prayers, mysteries of the rosary, how to go to confession, symbols of Church, and so on. Catholic magazines are especially good for pictures of saints, and our parish calendar has saints every year, so I use those as well.
Sower magazine for catechists has a centerfold in each issue with a famous work of religious art, and lesson plans for older and younger children.