A piece of food that is stale is dried out and old and doesn't taste good, and imagery is description using any of the five senses. Imagery is one of the basics of good writing, just as food is one basics of good health. However, just as food that is stale doesn't have much taste or nutritional value, the same can be said for stale imagery: it doesn't leave readers with much. Orwell states that stale imagery consists of
phrases tacked together like the sections of a prefabricated henhouse.
This statement about the henhouse is an example of vivid imagery: we can see a prefab henhouse—or a prefab house—in our mind's eye. Orwell also offers an example of stale imagery when he quotes a letter from Tribune:
The heart of Britain may be sound and of strong beat, for instance, but the British lion's roar at present is like that of Bottom in Shakespeare's A Midsummer Night's Dream—as gentle as any sucking dove.
The writer of the above passage has simply strung together a series of metaphors that are worn-out and don't even necessarily work together well. A clue is his use of images that don't have much relationship to twentieth-century life as the audience would have known it. This indicates that he is borrowing images from the past—meaning that they are old and stale.
Additionally, "cancer" would make a better choice than the archaic "canker," and "shriveling" would be more vivid than "atrophy." The auditory imagery of a lion's roar is stale and cliched. How often do people in everyday life hear lion's roar or experience a "sucking dove?"
In the last image, the writer overtly borrows from the past, hoping to look educated and authoritative by alluding to Shakespeare. In Orwell's opinion, none of this imagery took any work or thought from the writer, and, as a result, it conveys very little meaning.