Measure the size of your marble accents to get a rough idea of how big you need to make you art pieces. Use a computer to crop your selected images into circles of the right size and print them. I used card stock which is better for durability. You can use your own photographs or you can search this great site I found several months ago. They have tons of free vintage artwork for download.
Cut your pieces out. A tip for cutting circles neatly: cut further back on the scissors. This give you more control of your pivot action and keeps the cut nice and rounded.