inDesign: how to retrieve an image which is gone but yet present in the document?

In my document I used images which are now gone (files were deleted) but yet they are still in my document. When packaging the document inDesign says links are missing though. I don't understand this, if they're missing (I know they are) then how come they are displayed in the document? And what can I do if I don't remember where I found them initially?