Do you mean that display the body of the email in the Rich text editor and display the image in the body of the email?
If so, I am afraid that using 'Office365Outlook.GetEmail()' in Power App to display pictures in the body of the email is impossible,There are several reasons:
Office365Outlook.GetEmail(Gallery1.Selected.Id).body does not actually contain the content of the image (although it contains <img>tag, in fact the src parameter of <img>tag is filled by the ID of the image).
the pictures in the body are considered as attachments to the email and can be obtained through Office365Outlook.GetEmail().attachments.But this formula cannot get neither the URL of the picture nor the content of the picture (the value of the content attribute is empty). If you want to get the content of a picture in base64 format, I'm afraid you can only get it through flow.
Based on the above two points, I think the function you need cannot be achieved or is extremely difficult to achieve.