What a coincidence, OP. I'm reading "The Age of Manipulation", which deals with subliminal messages including hidden images.
Images can be inserted into other images to attract the attention of the viewer, fixate the image in the person's memory or inspire a response.
Sexual imagery is often used to draw the eye and bring attention. This can be achieved by inserting genital shapes, or words like "sex" on the background. One instance of this is the fact that I always walked with my friend past a book stand, and the first book that always caught our eye was one titled "Deep Throat" (related to whistle blowers, if I remember correctly). This works because sex is treated as a taboo in our society, and we are trained to forcibly ignore it and repress it. This serves only further fixate it on our subconscious.
A feeling or response can be achieved by the insertion of words, symbols or lighting effects. A person cloaked in shadows and placed outside of the center of the image inspires much more dread and suspicion than one depicted with emphasis in a well-lit environment (the same person, with the same clothes and stance, that is). Other words like "kill", "blood" and "hate" can be inserted to associate the object depicted with negative feelings, as well as more abstract symbols like distorted faces and skulls.
Perhaps I am steering away from your intended topic. Apologies.
Imagine a website wanted to increase traffic. They could add images or words that generate a response to the background in a way that you cannot see consciously, but your eyes are still able to notice it and feed it to your brain. This theoretically could increase interest on the page both by those who are new and by those who are already familiar with it.