For downloading single image or video :
For downloading a particular image or video , the extension adds a button below every video that can be clicked if you wish to download the video. The below screenshots shows the button which will appear below every video and image file. There are three options below a image , for downloading the whole picasa album , downloading the particular image and opening the image in a new tab window.
For downloading images in bulk
So , this extension takes your Google+ experience one step further.