"And now for this brief commercial [art] interuption."  

My old 1920's enamel advertising sign for cat food.
Fantastic double entendre value.

Nice
I am sooo tempted to mention flavors. I have worked in many old sign shops and the genuine, thick enamel signs have always been favorites with me.
Is no-one seeing the secret image in my poster? It's a fourth person.

Vintage Damage---
I must be blind-- where's the fourth figure?

I see a horse.....kind...
I see a horse.....kind of....The guy on top has his arm around its neck?

I see a map of British Hounduras,
the profile of painter Thomas Eakins, and the Stoning of Stephen (I can see the Apostle Paul standing to one side).
But I don't see a fourth trapeze artist.

I didn't say "trapeze artist"
The fourth person looks a lot like Punch or Judy. I swear that I am not effing with you and that this fourth is big enough to be seen without difficulty.

I found Elvis.
The cute 70's Elvis, not the fat, druggie Elvis.

You guys are all on designer drugs
. It would be unfair to future readers for me to give it away.
I would like to pay tribute though: This is someone who makes only a brief appearance but is likely to be remembered by all.

Osama Bin laden ?
Are you a member of a highly dubious and sinister religious cult by any chance?
Sorry, I still can't see the hidden message, would it help if it put on a pair of the old school 3D glasses?

I actually do see the face of...
I actually do see the face of a man on the right in the brown background, but I can't tell who it is. Maybe if the picture was slightly larger.

I see the guy below hidden...
I see the guy below hidden in the background. Now, I know the origin of the striped outfit he always wears. Who would of thunk Waldo was actually a polish circus performer?

Wait--
Is Waldo the bottom figure-- doing a split? Boy, these early "Where's Waldo's" are a lot tougher. He's barely recognizable in drag. Thanks, Jesgord.

Vintage
You should not be so disappointed. You are looking at the article in person, I am looking at a largish postage stamp. give some credit that we may not actually be blind and perhaps it is the medium (computer), how in focus your image is, and/or the size you posted. If it is a "hidden" figure, any of these, let alone a combination, could render the extra figure invisible.

I actually do see a large fac...
I actually do see a large face in profile on the right hand side of the image. The nose (What I previously thought was the horse's head) is located in the space between the left arm of the upper character and the negative space in the torso area of the middle character. The black shoe of the middle character (where the woman is holding on) creates the mouth.