When using an image to submit a form, the name of the image is not available to the script as a posted variable. Rather, the x and y coordinates of where the user clicked on the image.
Where a regular submit button with a name attribute of "submit" would be available to your script as $submit or $HTTP_POST_VARS["submit"], an image would be split into two variables,$submit_x and $submit_y, and $submit would not be defined.