By default, Evolution doesn’t display images in HTML emails. It’s a good security and anti-spam practice but it’s also nice to have the ability to override this behaviour. The Ctrl+I keyboard shortcut (View>Show Images is the way to do it through the menus) will do the trick on an email by email basis but you need to add the email address to your address book for a more permanent approach. There’s a little extra to make the latter work and it involves heading to Evolution’s Preferences dialogue box (Shft+Ctrl+S or View>Preferences) and selecting Mail Preferences from the sidebar. Clicking on Mail Preferences gets you where you need to be. The part of the screen that’s relevant is Loading Images and there are three options: Load images in email from contacts is the option that you probably want more than Always load images from the Internet because keeping Evolution’s anti-spam defaults is probably a very good idea. Apart from sender whose images you don’t want to see, you should now have images displaying in HTML emails.
Aside: The theme in use for the above screen capture was from Ubuntu Studio rather than SlicknesS, which is my usual choice. The latter makes the above screen unusable because the text cannot be distinguished from the background and it’s only for this tab that it happens too, a combination of posssible Evolution programming inconsistencies colliding with potential theme design gremlins in my view.