Hi Everleigh,
It isn’t disrespectful or a turn off to say thank you to guys who compliment your photos online. There is a good chance that the guy you are dating does the same thing with platonic girlfriends he has online.
As long as you don’t take your comments beyond a polite thank you, you don’t have to worry about turning off a guy. If anything, it normally inflates a guy’s ego when so many other guys compliment the girl he is dating.