I've seen this before, and can be a few different things.. Mostly nothing to do with Nintex Workflow..
Also I've been able to fix this before by ensuring the Intranet (Assuming that is where your images are stored) is setup as a trusted Site in IE. Outlook uses IE for its settings.
Then there are the obvious ones like are the URLs to the images right ?
yes. Images Urls..I have verified email settings and ie settings. they are fine. but unable to see the images in workflow notifications.