I came across the following exchange on a WordPress theme vendor support forum. I can't link as a login is required.
Client: Hi, I am trying to get the background of this site to appear correctly on an iphone.
Vendor: Please let me screenshot from iPhone. I'll take a look.
Client: file:///Users/[username-redacted]/Downloads/photo.PNG Here you go.
Vendor: It's a local path to the file. Please upload that and let me an URL to your screenshot.
Client: If I put this in the http:// window the image appears. How else would you want it to arrive?