Mussels can sometimes be difficult to identify even in hand and with only a photograph it proves to be even more difficult. Having a photograph that shows all of the important characteristics for identification is highly important. The following document has detailed information on how to take good photographs.
http://www.tpwd.state.tx.us/huntwild/wild/wildlife_diversity/nongame/mussels/media/guide_photographing_mussels_howells1.pdf