I definitely know what the three white lines look like; I have a boy and a girl :-). However, because there's no way to tell for sure what angle the picture is at, it's hard to say. If it's from below, i.e. normal anatomical scan, it's a girl. However, if this is from above through the knees...I see a boy. So I still think it's hard to say.
here is the link to a distinct girl off the internt http://pregnancy.about.com/od/isitaboyoragirlfaq/ig/Girl-Ultrasound-Gallery/It-s-a-girl----18-Weeks-Twins.-ULT.htm
edit ~ what people are seeing that looks like boy stuff is the light colored shadow area between the legs but that is the insides possibly the cord but what you look at that proves its a girl is the very bright white two likes that resimble a hot dog bun. that are closer to the stomach than the shaddow.
girl to me. looks just like my daughter at 13 weeks. if it looks like a empty hot dog bun its a girl a boy would have a big ball and have a penis hanging out.
It looks a lot like a girl to me. I asked DH what he thought too and he said it looks like a girl as well.
hmmm I just looked at it again and now I'm not so sure. It does appear that what I thought was a penis is actually the umbilical cord but it's still hard to tell.