Showing much better than reality. To flatter means to lie exaggerating good qualities, for some specific aim; in this case what is left of the meaning is just the 'lie part'.

This means that when the lighting or angle of a photograph is so that the subject (person or thing in the photo) looks nice, or better than it/they would in a different light or at another angle. So if you can take flattering photographs, you will more likely be a better photographer since the person will appreciate how they or their subject looks!

Ellen and Andrew, thank you very much for your helpful answers.