Hi Kelly,
In reading your question, it sounds like you are going to use a belt buckle blank as opposed to a belt buckle mold, yes? I’m writing my answer below like you are using a blank.
First, seal your item with a clear drying white glue, including the edges, then gluing the image to the bottom of the buckle. Seal again inside the bezel. You definitely don’t want to experience any water stains. This is the one I use: https://shop.resinobsession.com/collections/tools-and-supplies/products/ice-resin-paper-sealer
I would not pour resin first, then put the image down, because you don’t want to chance trapping bubbles underneath your picture that may come up later during curing.
As for the resin, use the Resin Obsession crystal doming resin: https://shop.resinobsession.com/collections/resin/resin-obsession-crystal-doming-resin
While this isn’t a belt buckle, this tutorial on our blog should help as well: https://www.resinobsession.com/resin-tutorials/how-to-make-a-resin-pendant-using-a-bezel-pendant-tray/
Good luck!