The image URLs must be direct links to the images. URLs such as Flickr links, which are loading the image "inside their site", but without being it an explicit link for the image itself, won't work. This happens with a lot of different image hosting sites. Dropbox or Google Drive also load an image viewer instead of returning the image itself.
In Imgur for example, you have the "direct link" and the "share link". The share link loads the image inside the imgur site, so it won't work in the infowindows, but the direct link will, as it just retrieves the image resource.