Sometimes I think I see something moving in the darkness, but later I realise it is just my imagination. Sometimes I think I remember something, but then I realise it is just my imagination again. If the stories of recovered memories are to be believed, sometimes people think they imagine things, only to discover that these are actually memories. One step here is taken by Hume. For Hume images differ in their 'vivacity'.
Memories are less vivacious than 'perceptual' images, for example.
This is not quite an answer, yet. The step open seems to be to say that when I have had a 'perceptual' image, and careful investigation has revealed that it was a 'perceptual' image, I remember what it was like. Next time I have an image I can then classify it. Hume's putative solution comes under a general class of 'label solutions'. Different images have different identifying characteristics, or 'labels'.