I think you mean "search image." "Search image" is a technical term in psychology and animal behavior. It is a noun phrase. "Search" is a noun being used as an adjective. A "search image" is a kind of image that you use when you are making a search. A "search image" is an image that you form in your mind, that has the characteristics of the thing you are searching for. You look at a scene and match what you see against your search image. If I am looking for a book, and I believe it is blue, I will form a search image of a blue book. I will look around the room and notice every blue book. If, unfortunately, I am wrong and the book is actually red, I may look right at it and not notice it because it does not match my search image.