Hi,
I don't think you can identify which image has been clicked.
you can do one thing take a hidden field in the form
ex: <input type="hidden" name="click_img" value="">
write the following in your <input type="image"........> tags
onClick="document.formname.click_image.value='s1'"
and for the second one
onClick="document.formname.click_image.value='s2'"
Now you can find out the value of $click_img to find out which image has been clicked.