Are there any differences between the two?
A Google search says that astronomy images is much more used, but I think that astronomical images sounds better.
I find astronomical images to be about twice as heavily used taking google results as the metric, and overwhelmingly so from an ngram comparison:
It is common for us to use nouns as adjectives, but it is restricted to some uses (though it is appropriate here) and is generally not done if an adjective form of the noun can be used. Hence we would indeed heavily favour astronomical images as the most appropriate form.
In the case of compound nouns, the opposite applies; it's permissible to use an adjectival form but more common to use the noun. Hence radio astronomy images would be more common than radio astronomical images, though some people's style does favour the latter.
While the above would indeed favour astronomical images very strongly over astronomy images, it is common to use a noun adjunct rather than an adjective in headline-style. This would indeed seem to account for many of those we can find on google, as they are frequently in titles like "ScienceDaily: Astronomy Images" and "Best astronomy images 2012" rather than in body text.
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I would understand astronomical images to mean pictures of the stars and planets. I’m not quite sure what astronomy images would be, but it’s not necessarily ungrammatical, since it is not at all unusual for one noun to modify another in English. I suppose it would mean images related in to astronomy some unspecified way.