the trigger code used for a submit type and an image type is not the same.
a submit type contains a value attribute, where an image type cannot pass a value.
with a submit button type, the trigger code will be written as:
if (isset($_POST["buttonName"])) {
where $_POST["buttonName"] contains the value of the submit button element.
an image element doesn't have a value, it passes an x and y coordinate of where the image was clicked, so the trigger code for a submit button would be written as:
$_POST["buttonName_x"]
if you change the button type, you will need to edit any server behaviors to re generate the trigger code, or manually change the code to include the _x