Images and tables can really only be added in HTML formatted email. The plugin uses the WP function wp_mail to send email, which sends plain text by default. To allow the plugin to send HTML email you need to filter the headers for wp_mail to be HTML.
WP-Members does have a filter hook for the headers for email that is sent via the plugin so that you do not necessarily have to make this a global change for your entire site. You could just filter the plugin’s email headers with wpmem_email_headers
http://rocketgeek.com/plugins/wp-members/users-guide/filter-hooks/wpmem_email_headers/
