For a fascinating (to me, anyway) discussion of whether a dragon is a Tawny Pennant or a Four-spotted Pennant, see:
http://bugguide.net/node/view/823025
http://bugguide.net/node/view/823025
Many dragons are distinctive and easily identified. Others have subtle differences. Some change color as they mature. Some immature males of certain species resemble adult females. Some dragons can simply not be identified from a picture alone. Here are some examples of how tricky it can be.