If you use a nodejs web-server to run your app, you can add this to karma.conf.js :
proxies: {
'/path/to/img/': 'http://localhost:8000/path/to/img/'
},
If you don't use or want to use another server you can define a local proxy but as Karma doesn't provide access to port in use, dynamically, if karma starts on another port than 9876 (default), you will still get a 404 error...
proxies = {
'/images': 'http://localhost:9876/base/images'
};
Based on this answer
Related GitHub issue for more info ;)
I would also suggest using requireJS because Karma does not deal well with fixtures...