When something is picture perfect it means that it is without flaws, or exactly as you imagined it would be. A perfect picture can mean the same thing, but only in reference to a picture (painting or photograph).

It is an expression that means it was extremely perfect. The difference between "picture perfect" and "perfect picture" is that "picture perfect" is saying how perfect it was while "perfect picture" is referring to the picture and not the degree of perfect. The expression is a bit redundant since perfect means there are no flaws but "picture perfect" is just over emphasizing that fact.