Thank you for the recommendation. I'll give that extension a try as soon as I'm back home. So far I'm using the native options of Chrome: chrome://settings/content/images. From that page I can set the default behaviour and the whitelist/blacklist of websites. When I visit a webpage, I can view the images by clicking on the small lock to the left of the url bar. I didn't find the option to unlock only one image, it's all or nothing.