[fxwoody]

Would it be possible to have this custom icon INSTEAD of the threadicon, instead of having it left or right to it?

You mean removing the thread icon totally?! Not sure if it would be good anyway.

Does not work on vB 4.1.2, the thumbnail URL bar does not appear.

Did you selected the auto add to template in options or manual???
If manual, then you might have to add this code to the newthread template:

<!---thumbnailurl--->           
                    <vb:if condition="$show['thumbnailurl']">
			<div class="blockrow">
				<label for="thumbnailurl" class="full">{vb:phrase thumbnail_url}:</label>
				<input type="text" name="thumbnailurl" id="thumbnailurl" class="primary full textbox" value="" maxlength="250" />
			</div>
		    </vb:if>
<!---thumbnailurl--->

after this:

&nbsp;<img id="display_posticon" src="{vb:raw selectedicon.src}" alt="{vb:raw selectedicon.alt}" />
			</div>

Tho, i didn't installed 4.1.2
Will get back to you with this