Trying out my new installation of the fantastic Evolution email client, I was a bit disappointed that images were not displaying in HTML emails that I receive.
Tinkering around, I found that images are easily displayed once activated in the program's preferences.
Edit->Preferences->Mail Preferences->HTML Messages
Under "Loading Images", select "Always Load Images from the Internet".
Be aware that this will cause Evolution to always load images from the internet; you will not be given the option on a per email basis.