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 from github and migrate the database:
cd /path/to/redmine/
git clone git://github.com/dkalachov/redmine_email_images.git plugins/redmine_email_images
bundle
rake redmine:plugins:migrate RAILS_ENV=production
The latest version of this plugin is only tested with Redmine 2.6-stable and 3.0-stable.