Very simply put, when guys post "shirtless pictures", it looks like they are desperate for attention. That, or they figure it will make them more appealing as a potential date or mate. (I even know females that prefer guys on the huskier side, and find muscles a turn off.)
I really do understand that some guys spend a lot of time in the gym and have put serious effort into sculpting their physiques; however, sometimes showing less is more. Women dig confidence, but going shirtless over the internet can inevitably have the opposite effect, reeking of insecurity and cockiness. It may look good (to some), but ultimately is not a good look~