Hello Whetstone,
I would recommend building a Custom Submit Button using a Text/HTML question and initially hiding the built in Submit button. Use this blog to catch you up to speed with this concept:
http://www.datasprings.com/news/blo...mit-button You'll then inject your Disabling Button JavaScript before the Dynamic Form Submit Processing procedure is called.
For instance, this will be similar to the OnClick event for your custom HTML Button(Text/HTML field with HTML):
onClick=
"
document.getElementById('HTML_Button_ID').disabled = true; __doPostBack('dnn$ctr2536$DynamicForms$lnkSave','')
"
Now the issue appears to be because you're using an Image Button for your Submit Button. If you must continue using an Image Submit Button.
I would recommend using an anchor tag wrapped around an img linked to the image you're using. So change your out of the box Dynamic Form submit button to an HTML Button. Save and Update Settings. Now copy the JavaScript from the OnClick event of the actual Submit Button of the Dynamic Form. Paste this into your OnClick event of the anchor tag in your custom HTML button
<a href="#" onClick="document.getElementById('HTML_Button_ID').disabled = true; __doPostBack('dnn$ctr2536$DynamicForms$lnkSave','') return false;"><img src="/source/to/your/image.png"/></a>
I hope this helps.
Also,anytime that a Postback occurs(what happens when the submit button is clicked), after the page reloads, the JavaScript to disable the button is lost, which re-enables the button.
Thanks,
Ryan