Equally, sorry to disagree here, but a dream is rather like a painting - there's really no such thing as an irrelevant element, everything present is there for a reason - it doesn't just appear simply because its a dream. Like marks on paper or canvas, if they're there they're there for a reason.
Hehe, well, I consider it depends on the author of the picture . Salvador Dali undoubtedly placed every little detail in his pictures for a certain reason and giving it a certain meaning, whilst Picasso (this is only an example... I think there are even better examples, can't think of one right now) has many paintings which were done without a certain meaning or the wish to reach some kind of visual climax for the viewer, but just randomly representing modern art (Yeah, I'm sorry to say this, but I think most Picasso paintings are a bunch of lines and colors done with total randomness and trying to look interesting . He was undoubtedly a genious in his domain, but then again, his domain was not limited in any way, since he could claim that his painting was meant to look like that, no matter who would have said anything against it.))