I think you may be misunderstanding the sequence here:
The form COLLECTS data that will be submitted to your server.
The form ACTION attribute identifies the server page that will process the collected data.
The SUBMIT button (in this case, masked by your .gif) is used to tell the page: User is done entering values, send the collected value to the ACTION page.
When the user SUBMITS the form, the ACTION is: done with the page the form is on; now send the collected value to the ACTION page.
There may be some slight delay between clicking the SUBMIT button, and the display of the ACTION page: whatever time it takes for the collected data to reach the server and for the ACTION page to process that data and return the new page.
You can indicate that the button has been clicked by using javascript code.
<input type="image" name="load" src="loader.gif" onclick=fixbutton(this);>
<script>
function fixbutton(mybutton){
mybutton.src='submitted.gif'; //you'll need to have an alternative picture --
mybutton.disabled=true; //stop multiple clicks;
}
</script>