Images are very concrete "word pictures" having to do with the five senses--touch, smell, taste, sound, movement, and especially sight. As Perrine points out, images make readers experience things vividly. To figure out the imagery in a poem, the reader should first make a list of every single mental picture, or visual image, that comes to mind as he reads the poem. He can then go back and find other kinds of ideas that have to do with physical sensations--sounds, tastes, smells and so on. Finally, he can go back and think about all the ideas these different images could imply--figure out their connotations, in other words.
For example, if a poet compares something to a ship, the reader might think about what ships look like, and then think about what it feels like to be on a ship. How do ships move? Where do they go? What sights, sounds, smells and sensations can we associate with ships and being on ships? After thinking about these questions, the reader can go back and attach these ideas that a ship implies to the thing to which the ship is compared, and finally try to fit these ideas into the overall meaning of the poem. See Emily Dickinson's poem "There is No Frigate Like a Book"
Importantly, poets often place images in opposition to each other. This creates what is known as "tension." Tension is often an important clue to the meaning of a poem; it also creates drama and interest and is a key to paradox. One should look out for strange contrasts in images in the process of analyzing poems, and think about the responses they arouse in a reader. Images can be part of similes and metaphors, though they are not always.