The halachah is clear (Shulchan Aruch, Orach Chaim 75) that one may not recite Shema, or any other words of Torah or prayer, while in the presence of parts of the human body that should be covered.
What about photos or drawings, though? For example, you may be riding on the subway, and you want to put the time to good use by saying Tehillim or studying Daf Yomi or Rambam or whatever - but you're sitting across from a racy ad. Or (inspired by this post at the Seforim Blog), the title pages of some sefarim on the table in front of you feature drawings of nudes or semi-nudes. Are these indeed considered the same as seeing them "in the flesh," or are they halachically insignificant, at least for this purpose?