en.javascript.info/2-ui/3-event-details/11-onload-onerror/1-load-img-callback/solution.md

The algorithm:

1. Make img for every source.
2. Add onload/onerror for every image.
3. Increase the counter when either onload or onerror triggers.
4. When the counter value equals to the sources count -- we're done: callback().