404 error?

I’m getting 404 errors on an image where no retina is provided. I assumed the plugin looks to see if there is retina, otherwise ignores it? Isn’t that the point? Or am I not using it correctly?

Usage: images in a theme (php template file). Activated plugin. Couldn’t find any other options.

Should I be calling the images in a different way (they haven’t been uploaded via Media, I am developing the site locally)

Thanks!