For anyone trying to use cURL to submit to an ASP/ASPX page that uses an image as the submit button.

Make sure that you have 'button_name.x' and 'button_name.y' in the post fields. PHP names these fields 'button_name_x' and 'button_name_y', while ASP uses a dot.

Also, as noted above, be sure to include the '__VIEWSTATE' input field in your post request.