Be proud of yourself, you have an eye for contrast. Please remember that you may take 100 photos and only get a handful that you really love, it's the way it works in photography. Sometimes it helps to stop looking at them for a while and come back, I know I have some that I didn't like at the time, but later on really fell in love with.
The cropping you're talking about at Walmart is called digital cropping, and instead of a true 4x6 it's a 4x5.something, you can opt out of that, at least I know you can online, however it still changes the cropping slightly.
Good luck, and keep working at it, a good photographer never stops trying!