In order of perferance:
Option A : Use the plugin that you mention because the footer can have it’s own custom sidebar, this makes it easy to manage. If you don’t like that plugin it still makes sense to use a custom sidebar for your footer, there nothing stopping you from writing your own widget or shortcode.
Option B : Create a CPT (or page), attach the images to it and then use WP Query with the paramater set to 'post_type' => 'attachment'. Please refer to http://codex.wordpress.org/Class_Reference/WP_Query for the parameters.
Option C : Query the image directly using wp_get_attachment_image_src($attachmentID, $imageSizeName);