Continue button (signup page) with .gif loader

Hi,

I hope you can help me, it's nothing too complicated :

I'd like to add a little .gif animation when a customer hit the "continue" button on the signup page just like this form : http://soundsfunapp.com/contact-us/ (when you hit the send button, there's an .gif showing up. You can try, it's one of my website).

I'd like to change to the "redirection page" as well. Instead of having the traditional "You will be automatically redirected to secure payment page in about 2 seconds", I'd like to remplace it by a .gif.

Since the redirection to the... redirection page can sometimes take a while, It would help me a lot to have this little gif showing up.

Thanks in advance and Sorry for my crappy english

Instead of adding the .gif, I tried to change the continue button value when clicked :

<script type="text/javascript">
function changeText(submitId){
var submit = document.getElementById(submitId);
submit.value = 'Loading...';
};
</script>

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<input type="submit" value="&nbsp;&nbsp;&nbsp;#_TPL_SIGNUP_SUBMIT_CONTINUE#&nbsp;&nbsp;&nbsp;" id="submitbutton" onclick="return changeText('submitbutton');"/>

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It works but I feel that the page takes longer to load now. Is there anything wrong with my script ? I just want to make sure that I'm not making any mistake... I don't want to "broke" anything.