Change the name of the first background image

Which image we pick doesn't matter, but our logic only looks for
the first one, it doesn't loop through all possible matches.
Don't assume the response will be valid JSON
Allow 404 here even though a 405 would be better

As agreed on https://github.com/mozilla/identity-ops/issues/31 this
should be fixed in code, not in nginx.
Change the POST error message as agreed on https://github.com/mozilla/identity-ops/issues/31
Add test requested by Gene on https://github.com/mozilla/identity-ops/issues/36

This never worked on production, so it should continue to fail.