As far as I know you can't do that. Still, here are ways.
First, in some browsers you may style form elements with CSS.
But if you want to use your images, you may try following (but it may be the pain in the arse to make it cross-browser) :
First, create images both for checked and unchecked button.
Then, smth like this:
<form>
<input type="hidden" name="poll" value="1">
<img id=1 src="foo.bar" onClick="select_opt(1);">Option 1<br>
<img id=2 src="foo.bar" onClick="select_opt(2);">Option 2<br>
<img id=3 src="foo.bar" onClick="select_opt(3);">Option 3<br>
<input type="submit">
</form>
and then write select_opt javascript function, that should do the following:
1. Change the current image from non-selected to selected (setting "src" attribute");
2. Change previously selected option from selected to non-selected image;
3. Set hidden field value to 1 / 2 / 3 respectively.