Placeholders for missing images

If a data source doesn't contain images for certain items, what is the best way to indicate that the image used is a placeholder image?

The type of placeholder image would probably alter dependant on the type of image that should be there. For example if its a missing image of a person then a silhouetted image of a person would probably be used. But what if its a generic image whereby I don't want to indicate the type of image that's missing, only that this particular item doesn't have its own image.

Any thoughts?

EDIT/ADDITION

Ok lets get into specifics then. Lets assume a placeholder is needed. it has to be a generic placeholder i.e. no human silhouette or identicon or file type. Is the best solution a variation on the flat grey mountain and sun icon? I believe it is but what are peoples thoughts on this?