The .lev file was pointing one of the images to a wrong directory and should have never worked.
However, one of the earlier versions of Snails ignored this error and just showed a blank picture, instead of not loading it at all.
The latest version of Snails checks the .lev file for errors, and if one exists with some of the images, it won't show up on the levels in the game.
It could be argued both ways, but since the original .lev file wasn't correct in the first place, I blame that first.
Irrelevant, it works fine on mine now