The 24-year-old caused a stir in the Twitter community after she posted the picture of herself wearing a low-cut tank top.
She had also threatened to quit following a string of critical comments by some of her 60,000 Twitter followers.
McCain complained that she was "just trying to be funny" and was considering deleting her Twitter account.
She wrote, "When I am alone in my apartment, I wear tank tops and sweat pants, I had no idea this makes me a 'slut', can't even tell you how hurt I am. This is why I have been considering deleting my Twitter account, what once was fun now just seems like a vessel for harassment."
However, she has now tweeted again to apologize.
"I do want to apologise to anyone that was offended by my Twitpic. I have clearly made a huge mistake and am sorry 2 (sic) those that are offended," the Telegraph quoted her as saying.
She also admitted "this has been an embarrassing experience but also a learning one".