Link to the site will be very helpful.
Without link to the site I’m guessing that the images that’re already there in footer might be some kind of flickr widget or instagram widget, if this is the case theme only pull the image via feed so just go update the image on that social site that host the images.
It can also be just a simple html that pull image from your own site put in via WordPress text widget, if that is the case we have to edit that text widget changing the image source.
It could also be gallery shortcode (enable via plugin), in this case we have to change the parameter in the shortcode and/or update the plugin setting.