"...So I buy old pairs of spectacles from junk shops and when I meet a dog in the park I put a pair of spectacles on him. Of course, he shakes his head and they drop off sooner or later, so that is why there are pairs of spectacles in odd places in my pictures."

Illustrator of The Oxford Reading Tree series, aka: The Magic Key

[Finally! Solved! This has been bugging me for years ever since I first started spotting these spectacles in my kids' reading books. And now I know.]