Thank you all SO much again for all of your fantastic suggestions!! I thought I might compile them into a comment just to stick 'em all in one place :D
- For NSFW and SFW versions of images (with different privacy settings on alternative pictures!) (@amymist)
- For drawings or references with alternative color palettes! (@Airlie)
- Drawings that have a transparent background and a full background (for example), but still the same subject. (@Silken)
- Progress pictures, or different iterations of the same concept. (@Kadrina) (@Kirbygal)
- And, alternatively, as this function sort of serves as a bit of a 'sub-gallery' (Galleries within galleries), I've been thinking more about pictures that are quite individual but still follow a narrative and would take up a bit of space - so, think, little comic strips that people don't want to post individually, but still want to be available OR to be read in an order!
Thanks again folks! Really crossing my fingers for this one!