Just use another BB code for this.
[imgs]URL[/imgs]
Editing
templates/main_post_form.html you could achieve that there is inserted non-prompted [imgs] tag.
<a href="JavaScript:paste_strinL('',3,'[imgs]'+prompt('URL:','https://www.minibb.com/forums'+'/'),'[/imgs]','')" onmouseover="window.status='{$l_bb_image}';return true" onmouseout="window.status='';return true"><img src="https://www.minibb.com/forums/img/button_image.gif" style="width:22px;height:22px" alt="{$l_bb_image}" title="{$l_bb_image}" /></a>
