Pic on lock screen gets bigger

I have no idea how to explain this, but it is driving me crazy. If I happen to put my hand on my phone when it is not dark (say I pressed the home button, screen is still locked, but I can see the picture on my phone), sometimes it will make the picture really big, as if I was focusing in on a pic. If I take a picture of it, it is the normal size. In order to fix the problem, I have to shut the phone down and restart it, sometimes the pic is too blown up to be able to "slide" power down, and I have to press the top button on the phone to shut it down. When it comes back on, the screen is normal again. Any ideas on why it does this?