8 sites to get free images for your next email design https://www.campaignmonitor.com/blog/email-marketing/2014/07/free-images-email-marketing-campaigns/
Responsive design options for navigation in emails https://www.displayblock.com/2015/08/19/responsive-design-options-for-navigation-in-emails/
A Guide to Bulletproof Buttons in Email Design https://litmus.com/blog/a-guide-to-bulletproof-buttons-in-email-design
Six Great Responsive Email Designs http://blog.lyris.com/email-inspiration-six-great-responsive-email-designs/