I decided I will do separate courses in Facebook and Twitter and if I need in future I can always join them in one for the purpose. I will focus on the Facebook one first.
I have been searching for some time now for free image sources and these days everybody are blogging about that. Some useful links you can find free images for your content:
If you are looking to send e-mail campaigns or newsletters: https://www.campaignmonitor.com/email-templates/#gallery there is an option to pay as you go (if you send mail occasionally).
Image by Robert S. Donovan