Personally, I did not find anything inappropriate in either pictures. I agree with Souro that Miley's picture can appear to be more 'sexual' in nature. Yet, I still find them both pretty ordinary pictures.
Miley definitely is trying to appear more 'grown up'. Her recent performances and music video also portray the 'grown up' look attempts. I do not blame her for it. She's in her late teens and its usually the time pop artistes broaden their fan base. She is not the first teen to be pictured this way on a big name magazine cover, and will not be the last.
The only reason Miley Cyrus sparked such an outcry is because she is Hannah Montana - the Disney channel star. Her core fan base is girls 8-14. She is their role model. So people are up in arms that she is trying to portray herself more maturely. Jessica Biel, Alyssa Milano and Melissa Joan Hart faced similar flak when they transitioned from teeny bopper images to all grown ups.
Daniel Radcliffe is the only male child artiste I can think of who received criticism in transitioning from teenager to mature audience. He received lot of flak for his role in Equus, when he is the face of Harry Potter. Luckily for him it never snowballed into something he had to publicly apologize for.
I think for me the double standard comes in with people's resistance to female stars maturing. They seem to take it more easily with men.
Also if their argument is sexualization of teens - the intent of both images is the same. Make the person appealing. In fact Tyler Lautner himself was embarrassed of the hype created over his shirtless scenes and hopes to never have to do it again. He was not expecting the fandom to go so overboard and become a sex symbol.
I know Twilight fans in their mid to late twenties who feel weirded out by it. Their jaws drop, they comment on how hot and sexy he is, they look up his pictures - and then they discover that he is only 17 and go OMG, this is so wrong in their head. I know many men who feel the same about Miley, they ogle like she is just another MTV girl and then feel guilty because she is only 17.