Imagery means mental picture.

Example:

The rain is raining all around, it falls on field and tree, it rains on the umbrellas here, and on the ships at sea.

Image & Imagery

Image = single mental picture

Imagery = a collection of mental pictures

Example: 

Image: The rain

Imagery: The rain is raining all around, it falls on field and tree, it rains on the umbrellas here, and on the ships at sea. ["Rain" by Robert Louis Stevenson]

Explanation: You can picture the rain falling everywhere, on fields, trees, umbrellas, and ships.

In summary, an image is a single mental picture, while imagery is a group of descriptive words and phrases that create a full, sensory experience in the reader's mind.