Image Button Downloader от Styx

A download button will show up when you hover over an image. Image must be at least the minimum size(default 300x300). Buttons will only appear when you hover over the image or the button which is located on the top right of an image.

This extension is quite aggressive, so it can work on sites where other extensions fail. However, the downside is that it may break certain websites, such as YouTube. I recommend enabling it only when you are downloading images, then you can disable it till next time.