I don't quite understand...
If that img tag goes onto the form, is the user supposed to click the image?
If so, how does this identify you as the affiliate? Are you sure the "transaction=C" is not supposed to be your unique affiliate ID?
If you really need for the user to enter their account username, and then you need to fill in their username as you mentioned, that's possible using a multi-page form. But it doesn't sound quite right to me.