He will tell you whether the tiles are rated for outside use. If they are not rated for outside use, all the sealer in the world won't help. If the tiles are rated for outside use, they don't need a sealer because the waterproof finish is part of the manufacturing process. You said the contractor told you he put on four coats of sealer. I do not mind using elbow grease if there is a positive result.
Elbow grease is too expensive to waste.
Try visiting a local arts and crafts retailer and ask for decal remover. If they stock it, you are home free. Pierce the seal of the decals by rolling over them with a wallpaper removal tool called the Paper Tiger. This is going to be a very slow process, so really let it soak in and work its way under the decals. You'll find both products at most good hardware stores.