Do not depend on external resources. These tests are run by us browser vendors inside our own internal testing systems. And they don't like external resources (because suddenly they move, or the server is down or something). Causes lots of noise and uncertainty.

Does it have to be an img? Can it be an img with a data-src then instead maybe? If not you can find quite a lot of resources e.g. in the root of web-platform tests. So you'd do `../../../../images/green.png` to get to http://w3c-test.org/web-platform-tests/master/images/green.png.

Yeah, a bit ugly but it works. But data-src would probably work well enough :-)