A lot of those pictures are probably of circumcised penises. As long as you can pull the foreskin back behind the head without pain it is normal. Many foreskins tend to creep forward naturally and indeed they probably function better during entry into the vagina when they are forward. The shoud move back and forward during sex and masturvation without pain though. There are plenty of instructions on how to stretch on the male anatomy board if you search for them.