Excellent question. By "bathroom" picture, I'll assume you mean the classic, self-taken photo of a dude: shirtless, boxers peering over the shorts, tanned like a plumb and face twisted in to a 'tough-guy' snarl. Some men do this because they think it will lure the ladies. And to an extent it works.

There are girls out there who don't care if Bathroom Photo Guy has a 6th grade education and thinks The Tooth Fairy passes for a legitimate comedy. She is blinded by a rip-roaring physique and relishes the chance to run her neon fingernails over his 8-pack. She prioritizes pecks and appreciates this photo submission. It's no different then a guy saying his salary is "way over $250,000" or writing his interests include yachting, hiking the Himalaya and brunching with Don Cheedle - it's meant to impress you. One woman's lust for perfectly sculpted man tits is another woman's delight in discovering that a guy has a Masters in Social Work.
 
Now, having said that, does this make the photo-bathroom guy shallow? A silly gym rat?  Does it make the woman who needs her man to bench press double his body weight vain?   Sure, maybe. But really, all it does is illustrate to women what's important to him. The online profile is a dating business card, and these guys think the stripped down bathroom pic  will do big business.
 
I'm guessing you're the type who doesn't think this is the most salient part of getting to know someone. And that's fine go on with your bad self; but don't come running back to me when you discover your new boyfriend has uneven nipples and a serious muffin top.