Newer email clients, including Office 365 offer a preview feature when you insert a hyperlink in the body of your message. I found that for many pages in our school website, the preview image wasn’t the one I wanted to see.
It seems that in most cases the preview image will be the first encountered in the body of the web page. This is similar to Facebook scrape behaviour. If you are already publishing pages that will be shared via Facebook, there’s a good chance you have thought about image placement already and verified that the first image found is the one you want users to see inline with your shared content.
In my case, I hadn’t considered which image would be shown as a preview. In order to correct the preview behaviour and present an image that was more appropriate, I placed an image near the top of the page and then used some comment tags to hide it from view. Now when anyone embeds a link to one of our pages in an email, the preview image is consistent.