Art is subjective by nature especially when you can't ask the artist what they were going for and are left to your own devices for meaning. The task of analysing the symbolism of a piece falls on the viewer themselves. A fairly daunting task as most of us look at a painting at face value, so lucky for us experts exist that can interpret and explain the meaning of a work of art.
Sometimes though they find something that doesn't require special expertise, just an open mind and a good pair of eyes (or at least in one incidence in the following, the ability to read a newspaper).
The meanings behind the hidden references or small touches found in art are as varied as artists themselves. Sometimes they put in an example of technical mastery, sometimes there's a spiteful message hidden within the work and sometimes they just want to have a laugh.
So if you ever happen to be able to see the following in person you can show off your knowledge of the art behind the art without a need for a double take, because you'll know just where to look.