What format do I submit in?

I have just received this e-mail:

Those pictures are gorgeous. I would love for you to send some to us. I will then submit them to our Art Dept. and see what they think. Thank you so much for sharing this with Challenge Magazine!

My question: Since they have the link to my gallery, what format should I make a submission in? Do I use e-mail submission, disk, hard copy, or something else?