First, the picture was supposed to be of her friend and her friend's boyfriend, so why would they want you in it? Wouldn't it make sense to cut you out?
Second, you are being too suspicious, in my opinion. Just because she called it a night doesn't mean she secretly snuck off to a beach to go swimming. Even if she did, maybe it was because they wanted to go swimming and you didn't? There were three other people there. It's democracy in action. 3 votes trumps 1, sir.
Third, if you get the feeling someone doesn't like you, probably best not to be clingy. If she texts you eventually, so be it. If you're interested, maybe you ought to pursue her (if she lets off the impression that she is still interested in you), or if you want to go ahead and make a move and text her, that might work as well; but if she doesn't text and you don't text her (or if you decide to text her but she doesn't respond): well, that's the reason they invented first dates, I guess.
Live and learn, and let go.