Remote images display properly as long as the message includes the full url including the http:// in the image tag.
However, some people, and this includes the popular ZenCart don't do that. Instead they have something like this in the html portion of the message
http://www.domain.com/" />
Then for the image they do this...
In other email clients such as Thunderbird and Outlook this works. In RoundCube it does not.
To reproduce this, goto any online store that uses ZenCart sign up for an account or buy something, then try to read the email with RoundCube.
I am running version 0.5.2 on CentOS 5.6 with Apache 2.2.3