John, you need to upload the picture, not link it from your Atlanta History Center mailbox. The image URL has an HTTPS (secure HTTP) URI scheme which prompts us for a login when it attempts to show the picture:
"https://mail.atlantahistorycenter.com/exchange/John.Manion/Inbox/No%20Subject-26.EML/1_multipart_xF8FF_3_Atlanta%2010-08%20JUL%20018.jpg/C58EA28C-18C0-4a97-9AF2-036E93DDAFB3/Atlanta%2010-08%20JUL%20018.jpg?attach=1"