The plugin address this issue: http://www.redmine.org/issues/2770.
Email clients are not logged in and can't load images in email notifications from redmine. You can either allow downloading attachments for anonymous users or include images as attachments in email. This plugin uses second approach.
To install the plugin clone the repo
cd /path/to/redmine/
git clone [email protected]:INSANEWORKS/redmine_email_images.git plugins/redmine_email_images
bundle install
The latest version of this plugin is only tested with Redmine 5.1