If you are making an introduction to an essay, I would not use "imagine".
When introducing anything, you have to write it so that all readers of your work can relate. If you begin it with "imagine", you may write something that the reader can not imagine because they either don't know was it is or have never seen it before.
If the reader doesn't know what they are supposed to be imagining, they could feel a disconnect between them and your writing. And therefore, they lose interest in continuing to read.
Even though you may be writing an assignment for your teacher and they know what you are talking about, write the essay as if you are explaining it to someone that was locked away from all outside associations that way you have complete detail and aren't too vague with what you're writing.
If you need alternative beginning starters, let me know. I might be able to help. Also let me know what your essay is about or what kind of essay (book report, informative) because that could also determine what kind of sentence starter you'd want.