Feathers I've never used the photos app. However, I just followed your post, opened my email, selected one containing a picture. Opened the attached pic using browser (Edge) then I selected "File Save As" selected C:\Pictures. Opened the Windows Photo app and the picture was in the photos app along with other C:\Pictures for viewing. It works for me.
The only difference in the procedures that I see is I used "File Save As" in the browser. You mention that you copied the email att. pic into C:\Pictures.
Try opening in your browser > and use "File Save As.