They are completely different. Imagination is a mental capacity and a human ability, similar to creativity. Imagery is a stylistic feature of literature, involving similes and metaphors. For example, if the emotion of fear is depicted as a black cloud in a poem, this is a form of imagery.

In the most simple terms: Imagination = The pictures and thoughts in your head. It's the noun for the verb "to imagine". To make an image in your mind of something. Imagery = An element of writing or speech. It's the use of words to form an idea with lots of description. In essence, to create an image with words.