Showing thumbnail image on Facebook

Okay, website is complete and looking good .. went to share on Facebook and was hoping it would show picture (header image or featured image) with link but did not. First let me say I am not a developer. I found this online while trying to troubleshoot but am worried about fiddling with the code. Any pointers?

https://www.ehow.com/how_4938148_thumbnail-show-up-facebook-share.html