Input type="image" only works in Safari, but not IE or Firefox?

Question

Having accidentally discovered a form on a site I am working on was only working in Safari, I've just spent about five hours retracing my steps, before eventually discovering that the code that was making the form fail was my submit code.
The page I was looking at had quite a bit of validation, CAPTCHA code and some jQuery to style some drop downs, so I figured something there was the issue.
But it turned out just to be the submit button.
I've just tried it on my contact page, and its failing in Firefox (Mac) too (and so I assume IE as well.)
Should this work, or is using input type="image" on a form submit button a big no no?
This code works:
<input type="submit" name="KT_Insert1" id="KT_Insert1" value="Post your query" />
But this code does not work (except, it seems, in Safari):
<input name="KT_Insert1" type="image" id="KT_Insert1" onmouseover="MM_swapImage('KT_Insert1','','../images/buttons/post_enquiry2.jpg',1)" onmouseout="MM_swapImgRestore()" value="Post your query" src="../images/buttons/post_enquiry1.jpg" border="0" />
Or am I missing something here?

Answer 1

Thanks for that - might have realised this a lot earlier if it didn't work in Safari!
I assume I just need some CSS like:
.formbutton
  background-image: image1.jpg;
.formbutton :hover
  background-image: image2.jpg;
And:
<input type="submit" name="KT_Insert1" id="Send" value="Send message" class="formbutton" />