You can find quartz crystal caps (cabochons) that are usually used
for gluing to opal to make doublets and triplets. I believe there is
a glass cab supplier in England, but I've lost the link. Two quartz
cap suppliers that I know of are opalsmith.com and alpha-supply.com.
In rounds, they are available only up to 18mm. in diameter. You will
probably find that using these convex caps will distort your photos,
even using the "low dome" style.
I use 10 and 14 mm. high dome sizes for exactly that purpose
(magnification). These caps also have to be glued to their support,
as their back surface has a matt or satin finish (which completely
disappears when using epoxy, to become perfectly transparent). You
may want to look at watch crystals, which are available in literally
thousands of sizes and styles, both in organic (plastic) and mineral
(glass, sapphire, quartz, etc.) These are perfectly round, exactly
the same size and thickness, very hard, and have no visible
distortion. They also can be found in slightly convex shapes of equal
thickness to prevent distortion. Unfortunately, I don't have a link
for you for a U.S. supplier, but that shouldn't be hard to find. If
you use watch crystals; think about making your bezels waterproof.
I've seen pendants with photos (and other things) under glass get wet
and the photos wrinkle up. Not pretty. Hope all this helps.