Similar meaning, but less formal:
* Visuals
* Pictures
* Illustrations
* Descriptions
* Vivid language
* Figurative language
More specific, depending on the type of imagery:
* Symbolism (for symbolic imagery)
* Metaphor (for metaphorical imagery)
* Personification (for personified imagery)
* Alliteration (for imagery created through sound)
* Visual poetry (for imagery created through visual elements)
More abstract, emphasizing the effect:
* Evocation
* Impression
* Representation
* Depiction
* Conjuring
Ultimately, the best substitute for "imagery" depends on the specific context and the nuance you want to convey.