This is a topic that comes up a lot so you'll have plenty of responses I am sure.
When I started out (less than a year ago) my SIL was an SU demo and after I put the images on my first set and then complained to her that they didn't stick to the blocks, she said 'Don't bother with them. They don't work well. I learned that the hard way." So I never use them. Without them, my clings stick just fine. If you have some without the image stickers and they aren't sticking, I don't know what the problem is. No doubt some experienced demos will see this thread and be able to troubleshoot this with you.