I'm a photo-editing hobbiest, and I think I have a way of easily finding the letters in the photos. From what I've seen, Hershey has put white letters on a darker background. If this holds true for all the photos, all you have to do is run the photo through a threshold filter. This just makes the image black and white: the white letter will remain white while the darker portions of the image turn black. The only way the letter would not be clearly visible after being run through a threshold filter is:
1.) The letter isn't white
2.) The letter was on a light background
I think this is the photo you're having the most trouble with. Here's what it looks like when you run it through a threshold filter: I think rascule got it right.