if i were you id look into the main themes from the play you are designing for and try show that through the photo. also, the examiners always love symbolism.
what play are you doing? because, if for example you were doing 12th night, which is what im doing for coctume, you could get a pic that shows the theme of 'the uncertanty of gender', perhaps a half female half male. that isnt that great an example, but you know what i mean.
or you could choose what you think is the defining scene in the play, the scene that shows what the play is alla bout and take a photo of sum1 acting out that scene in costume. id say make it more conceptual than realistic and basic.
hope i helped.
oh! and go to the board of studies website if you havnt because they show examples of the best works for 2004.
