My two cents...
I think it's rude to bypass someone taking pictures if all you accomplish with the bypass is to fill a gap in the line.
IMO the polite thing to do is:
1) Just wait patiently.
2) If the photo taking gets so long that the end of the line is no longer visible, then politely point out to the picture-taker that the line has moved. That's probably enough to either spur the person to wrap up and move forward, or to tell you to go ahead of them.
And yes, I really think the onus is on the person BEHIND the picture taker to ask (politely) if they really want to go around, IF it's just a matter of filling a gap in the line. Think of other situations: if two people are talking in line and don't notice the line moving forward, and a gap appears, what do you do? Do you just go around them? Or don't you, at some point, politely point out the line has moved on?
If the line has disappeared from sight, then the onus is on the picture-taker to either wrap it up or indicate that people can go around.