Will a button-up shirt work? What colour? I tried a white polo shirt but that didn't seem to work.
Also the jacket will obviously be opened.
Can you post the picture directly to the site?
Personally would say its too informal for a polo shirt and so certainly wouldnt work with a more formal shirt.
All the styling photos you see of bomber jackets the model is wearing a t-shirt under it and thats what I'd also consider more appropriate.