Let me explain the concepts behind the examples others have mentioned.
What you should know by now is that imagery, as in the examples you mention, is used to create, as the term suggests, images in the reader's mind. Obviously, one applies different techniques to achieve the desired effect. The question you should consider is whether your attempt is effective. Does the image that I have created encapsulate the essence of that which I wish to share? Is the image apt and does it make sense or is it so illogical that it would be impossible for the reader to imagine or understand?
Furthermore, the simplest images are, most often, the most effective. You should, therefore, not attempt to create such convoluted images that they will spoil your attempt. In addition, the image you create should obviously share a resemblance with the scene you describe. You cannot, for example, compare a cat to a mountain without creating a link between the two. Look at some examples from Shakespeare for inspiration.
In Macbeth, for instance, Lady Macbeth instructs her husband to
"...look like the innocent flower,
but be the serpent under't."
The contrast between the two images is quite evident. The simile suggests that a flower is harmless whilst the metaphor alludes to the malevolence a serpent represents. In this instance, Shakespeare juxtaposes the one with the other, which adds to the effect.
In Romeo and Juliet, Juliet on taking leave of her lover sighs:
"Parting is such sweet sorrow ..."
The oxymoron proposes that taking leave of Romeo is both pleasurable and painful at the same time. It is painful because she wants to be with him and does not want to go whilst it is also pleasurable since it is he, the one she loves, that she is taking leave of, not some stranger or despicable character. Anything having to do with Romeo is pleasant, even if it is to only say goodbye.