To attach a flat-backed finding
The strongest bond would come from using a drop or two of resin. You can also place the finding into the resin as it sets up. If you don't want it to sink down into your resin, you can wait for about 1 hour for it to thicken up first, testing the thickness with your mixing wand or a pin.
If you're not mixing up a second batch of resin, you can use a strong adhesive such as E6000 (although it's stinky), or ZapAGap, a great super-glue.
Pour resin before or after adding elements?
When using molds, you should always pour your resin in before adding any items. If you put your embedments in first - especially photos or other flat pieces - you risk trapping air pockets under them as you pour in resin. This can result in rough finish, or lots of bubbles making their way to the top as the resin cures.