Cries every morning

My son has been going to daycare since he was a baby.  He's now in preschool.  It's a wonderful school and every day when I pick him up he tells me how much fun he had.  The problem is that ever since he's been about a year old, he cries when I drop him off at daycare/preschool.  It's not getting any better.  In fact it's getting worse.  He cries and grabs on to me and says "Mommy, don't go.  I want to go with you.  Please don't go".  It breaks my heart and it makes both of our mornings start out really bad.  The teacher is great and tries to help when I drop him off.  She even suggested I bring in a picture of me that my son can keep with him during the day.  That helped for a few days but now he seems to be going back to his old ways.  I don't know why he acts this way since he really enjoys school.  Any advice???