Dark spots!

I have made four attempts to make a simple coaster using resin and an printed image. The first one had major bubbles because I put the image down first and couldn’t see the bubbles until I released the coaster, so I started putting down a base of resin and putting the image on top of hardened base. Bubble situation solved! The second was ruined because the image floated up on one edge as the top layer was curing. The images were printed on an ink jet and remained clear and perfect. I had sealed them using Patricia Nimock’s acrylic sealer, being careful to also seal the edges once I cut the image out, also with the acrylic spray. The third time, I glued the sealed image down to cured resin with Mod Podge (letting the Mod Podge dry for 24 hours), then put the top coat on. The entire surface of the image came up with small dark spots. I thought maybe the Mod Podge had somehow caused this mottling, so on the fourth try, I just sealed the edges of the image (still using acrylic spray to seal it), then stuck the image down with a thin layer of resin. Once I knew it wasn’t going to float up, I put the last layer of resin on. Dark spots AGAIN! The images were perfect using just acrylic spray, and it seems like once I introduced Mod Pod into the equation, things went wrong. I don’t see how only using Mod Podge to seal the edges could cause mottling in the center of the image, so I can’t be sure it was the Mod Podge, either. Any suggestions? I’m about to just give this up as something I simply have no luck with, but before I do that I wanted to find out if there’s any remedy for this issue.