the images theory was a more general outgrowth of atomism, meant to explain the way the senses work, rather than focused on excusing hallucinations
There are several types of “images”: There is what the physical eyes can see. There is what the mind's eye can see upon making the conscious choice to do so (what we now call imagination). There is what the mind's eye can see without making the conscious choice to do so (what we now call intrusive thoughts, flashbacks, etc). And finally there are dreams. Because Ancient Greeks might have involuntarily seen pictures seen in their mind's eye, which they knew are not real (pseudo-hallucinations), there was a requirement to explain where those came from: they didn't come from the mind (because they were involuntary), they didn't come from reality. So they must have been "floating around" because their "particles are very fine and smooth" and happened to get themselves attached to the mind's eye by chance.