Now, this one has me genuinely stumped: How do images in our minds of the gods differ from images of unicorns and centaurs? Why would the former be considered real and the latter false and a combination of images colliding in the air?
I would say there that once they get to our minds they appear the same to us, and so it is up to our minds to be able to judge whether they reflect something real or something that results from those random combinations arising through "chance." I suspect Epicurus would say that the primary and even relatively "easy" method of distinguishing (or judge the faithfulness of to the facts) images that reflect real objects and images that reflect random combinations would be their "repeatability." You would generally expect that images coming from real objects will be observed over time and in varying conditions and are thus repeatable, while images arising from random combinations would be unlikely to be repeated in substantially similar form.
At least that would be the starting point of the way I would approach it, which is similar to the way we should approach all sorts of distortions and illusions, as discussed at length in Lucretius Book 4.