Sometimes while editing you may come across a page where an image has been deleted. If you want to recover this image you need to go the info button and select the images tab. There you will find a button labeled Page's deleted images which will allow you to bring back deleted images.
Note that the set of deleted images are occasionally all cleared out, similar to "emptying the trash" on most desktop computers. There is no set schedule for this, rather it is a function of how much resources they are taking, how old they are, and what kind of images (copyright violations, pornography) have been deleted recently. Images that are actively listed in the Images area (as well as their older versions) are not removed in this fashion.