[racket] reading an image from a file?

Silly question: How can I read a file and treat its contents as an image?
eg, I'd like to say something like:

(define cat-image (read-image "cat-image.png"))

The immediate advantage of doing this, rather than Insert|Image is that I
can keep my .rkt file in text form, so I can use other tools on it.  And of
course if I want to find an image dynamically I need something like this.

I can't even see what datatype a value defined by Insert|Image is.

--Mitch
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