Embedding inkjet prints (which I'm pretty sure is what Walmart photo station prints are) is a little challenging. The resin can not only liberate the dried ink but also the paper itself can change color as it saturates in the resin.
Better to not let the inked paper touch the resin directly. Get a spray can of flat clear coat. I have used Krylon but any brand should do. Put the picture down on an old cardboard box or some other surface that you don't mind being coated in overspray, and give it a very light misting from farther away than the recommended 6-12 inches. You aren't trying to get it wet with this spray paint as this can distort the ink too. Instead, use multiple super light layers to build up a protective coat on the front, letting each layer dry before applying the next. Once you feel that the front is sufficiently sealed and protected, repeat the process on the back. You can be a little heavier when doing the back because the front coating is now holding the ink in place, but don't go nuts and drench the back in one shot. Once you have coated both front and back and after that coating has been given more than recommended time to dry, you should be able to embed the coated image in resin.
At this point what you are doing is embedding the hardened clear coat in the resin. The inkjet photo is just along for the ride.