Need help with avatar: "The uploaded file is not a valid image"

I would like to change my avatar to a photograph that I downloaded onto my computer from the internet.

I want to make a photograph of Marilyn Monroe wearing a potato sack dress my avatar picture. So I saved the photograph to my computer, but when I tried to upload the image to be my avatar picture at thetruckersreport.com, I received an error message from thetruckersreport.com saying "The uploaded file is not a valid image."

Here is a link to the internet website that has the photograph that I want to make my avatar picture:

Hollywood Flashback: Marilyn Monroe Posed in a Potato Sack 70 Years Ago

Why won't this website let me use that photograph as my avatar picture?

 

Done. Thank you