Where does that image file reside? Is it a local file or is it on a web server? I seem to recall something about images tags in Manager templates (and in invoice notes) having to have http:// sources, and not file: sources. If your image file is stored locally, try uploading it to a web server and changing the img tag in your template to source it from there instead. That might do the trick.
Otherwise, please post the relevant part of your template here, and perhaps someone can figure it out.