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.

SIMILE.  A simile is a figure of speech comparing two unlike things, often introduced with the word "like" or "as". Example: "A beautiful sunset is like the twilight of a life fulfilled."

METAPHOR.  A metaphor is an analogy between two objects or ideas. Example: "The beautiful sunset is a final farewell of the passing day."

OXYMORON.  An oxymoron (plural oxymora or, more often, oxymorons) is a figure of speech that combines normally contradictory terms appearing in a range of contexts. Example: "The beautiful sunset birthed a new evening."

JUXTAPOSITION. A juxtaposition is synonymous with contrast; two objects or texts that oppose one another. Example: "The memory of the beautiful sunset burned brightly in his mind."

Simile: Driving away from the city and into the desert, Adam watched the sun in front of him dip below the horizon like a candle being extinguished.

Metaphor: The canvas of peach, pink and purple coated the sky as the sun dipped beyond the mountains in the distance.  This compares the colors in the sky to colors mixing together on an artist's canvas.

This sounds like it could either be an assignment, or you might be writing something and are looking to make it creative. I hope the latter as we editors aren't here to do homework for you but give you guidance in how to get creative assignments done.  I will let another editor tackle juxtaposition or oxymoron.