Hi Shiri,
Based on your description, I did a test. When designing the workflow with SharePoint 2013 Designer, the field types of a rich text column are Plain Text and As String, like the screenshot:
When I used Plain Text in the email body, after the workflow ran, the email didnt contain the image from the rich text column.
Then I tried to use As string in the email body, after the workflow ran, then email also didnt contain the image from the rich text column. However, it displayed To always show content from this sender, click here. above the email body, like the screenshot:
After clicking the second click here in the above image, the image from rich text column could be displayed well.
So, please use As String for returning the filed when lookuping the rich text column. And check if the email also has this above message. If yes, click click here.
Note: you must make sure the recipients have proper permissions on the list and the library which stores the images.
Thanks,
Wendy