I thought it was a butterfdly in shape, but it can also be a double eagle I guess. too blury to judge... 
oh, alright. ;-P
but...
while not ruling-out the possibility it could be a butterfly, which is it more
likely to be, a pin w/ the Romanov double-headed eagle or a butterfly? remember, she
is a member of the House of Romanov, she
is in court-dress for the Russian imperial court (the double-headed eagle is associated with both) and it was a (relatively) common practice for their heraldic symbol, the Romanov double-headed eagle, to be a motif on items such as cigarette cases, cigarette boxes, rings, cravat/tie-pins, hat-pins, hair-pins etc. and while butterflies were also a (common?) motif as well, i can't recall ever having seen her depicted w/ butterflies in any form -- be it a piece of jewelry, a figure painted into a portrait with her, or on her clothing as a decoration or in the design on the fabric.
besides, doesn't it just seem more appropriate that it would be an enamled or jewelled pin with the double-headed eagle of her House and her Home that secures her veil, especiall given that she
is in the ceremonial court-dress of the Imperial Court of the Russian Empire?