I made a set of Cornhole boards with pictures on them. We used double sided tape but pictures weren't staying down. We modge podged over them next and that didn't hold them down either. All of that said after I applied my resin (not your brand) it didn't go even and there are air pockets under the pictures. I need to remove the resin and completely redo it I think. I figure the easiest way to do that would be to use a sander. However I am very unfamiliar with that and what to do with it. If I sand it what type of sander should I use? If there is a different way of doing it I'm open to that as well.
It depends on the size of area you are wanting to sand.....if its more than what you are willing to do by hand w/ sand paper then I would suggest a small hand or palm sander from any better home improvement or hardware store.
Orbital/palm sanders use sanding sheets 5" in diameter, so if the area of the picture is less than about 12" X 12", this might be a better choice, but remember to feather the edges so there are no high/low spots or sharp edges.
To adhere the images to the wood, try some indoor/outdoor PVA glue or some contact cement. Or even Polycrylic. Test before use on the boards.