php form - optional fields

I have used a wizard to design the form below:

http://www.announcewedding.com/form1gold.html

Some fields are optional and allow choice of completion, but with the 'browse for image' field it does not and will not 'send' until an image is attached. This field, however, needs to be optional. How can I make it optional?
Does this explanation help? Otherwise, do you know of any good php form wizards?