OK, just change your code to something like this:
<script>
function submit() {
return confirm('Do you really want to submit the form?');
}
</script>
<form onsubmit="return submit(this);">
<input type="image" src="xxx" border="0" name="submit" onclick="show_alert();"
alt="PayPal - The safer, easier way to pay online!" value="Submit">
</form>
Also this is the code in run, just I make it easier to see how it works, just run the code below to see the result:
function submitForm() {_x000D_
return confirm('Do you really want to submit the form?');_x000D_
}
_x000D_
<form onsubmit="return submitForm(this);">_x000D_
<input type="text" border="0" name="submit" />_x000D_
<button value="submit">submit</button>_x000D_
</form>
_x000D_