The High-Shine Glaze in the spray bottle is AWESOME for sealing your images BEFORE you put your epoxy or other sealer on. It will keep your images from smearing or running if you printed them yourself. If you're using stickers you don't have to worry about it.
Now, if you're okay with the "raw" edges of the paper showing on the sides of the cap - you can just glue it to the cap and be done.
BUT the application of either epoxy or one of the other methods gives the cap a much more finished look: it will be a bit elevated, almost like a little dome, and should also cover the edges of the paper and go over to make a totally smooth transition to the cap (still with me? LOL hard to explain, but if you're looking at a cap it makes sense)..... at times you might have to apply 2 coats of epoxy to seal the edges, but it insures that the image will not come off the cap.
Hope this helped a bit,