Hi,
if you submit a form with an image input then the only reliable post variables are <name>x and <name>y (in your case submit_x and submit_y).
Firefox additionally posts a variable with the name itself (in your case submit) if the value property of the input is set. Internet explorer only sends the x and y variables.
You can easily check that by adding print_r($_POST) to your code.
Change your script to check for submit_x or submit_y.
Thomas