I have always felt that the best description of imagery is to see it in the action of literature. When one sees imagery, they might be best to understand it. Bearing this in mind, I think being able to survey the literature that you have experienced for examples of successful imagery would be extremely worthwhile and compelling. For example, reading Wordsworth's poem, "I Wandered Lonely as a Cloud" is one such poem with different uses of imagery, or mental pictures. His description of the daffodils in the first two stanzas help to mentally evoke the sensation experience of seeing a field of flowers. Examine these stanzas and find the lines or verses that reflect sensation, as if one is experiencing the moment with the speaker. At the same time, these lines help to convey extreme emotion, and more if conveyed in the third stanza when the speaker seeks to make sense of this moment in "his mind's eye." I think that this is one poem of many where examples of imagery can be experienced for oneself.