I’m not completely familiar with WP but it looks like each widget is in a div with a class of .widget-wrap.
Assuming that to be the case and you want each widget to have an image across the bottom then i think the best option would be to add extra padding-bottom large enough for the height of the image and then use a bg image and use background-position to get it into place.
Perhaps creating a special class for this might be appropriate in case you only want it to apply to specific widgets.
