Thanks to dictionary.com:

verb (used with object): to place close together or side by side, especially for comparison or contrast.
Example: To juxtapose a hummer and a voltswagan beetle.

Surrealist artists juxtapose realistic images into unexpected settings to create a new, surpriseing world. They sometimes try to express the hidden conection between completely, supposedly unrelating objects. By placing impossibly differnt objects together, this a way of juxtopose.