I don't think the underage factor is as important as the indignity of the violation when someone masturbates to photos of someone who would likely not consent to the act.
The expression of consent can be tacit. Women in pornography, or who pose in sexually suggestive in magazines (I'm thinking Maxim and the like), are offering themselves publicly in a sexual way. They can expect to be taken as such.
What about the fact that Palin came in second for Miss Alaska? She obviously loves all eyes on her.
Miss Alaska is a beauty pageant, not a peep show. It is possible to separate beauty and sexuality, even if they're often tied together.
But let's take for granted that Miss Alaska qualifies as publicly offering oneself sexually. That was what? 20 years ago? More? That has no bearing on how one is to treat her today. Again, accepting that a beauty pageant is a sexual context, it might be acceptable to masturbate to beauty pageant pictures. However, entering oneself into a beauty pageant would never count for being seen in that sexual context (which I don't think it is) for the rest of one's life.
She does not offer herself that way anymore, publicly, and that does change the rules.
As for fantasies, you can't really help what fantasies you have, but I do think it's disrespectful to indulge in them if you can be confident the person about whom your fantasizing would not approve of what you're doing.