So how to interpret this statement：'How do you think these girls were heroes? ' I really don't understand what it means.

This is part of a conversation between two people. I will call them the "speaker" and the "listener". If you read between the lines, the speaker is saying that these girls are not heroes. The listener thinks that they are heroes. The speaker implies that the photos show absolutely nothing that would support them being heroes. Quite the contrary, the photos show (in the opinion of the speaker) that they are not heroes. He thus disagrees with the listener and challenges the listener to come up with some justification for calling them heroes.

Or can I understand it this way: 'Look at the photos. How were these girls heroes?' Is this understanding correct?

For me, each of these sentences are saying something slightly different. Sentence 0 "Look at the photos. How do you think..." is similar to asking: In your opinion, in what way(s) are these girls heroes? ________________________________ Sentence 1 "How do you regard the matter..." is like asking: What is your opinion on these girls being considered as heroes? (Do you agree that they should be seen as heroes?) _________________________________ Sentence 2 "How do you think these girls became..." is like asking: What do you think these girls did to be considered as heroes? But this is just my perspective.