Had this come up in the comments of some image, I think there should be a field to add next to a source that allows you to enter the “originally posted date”, or perhaps having this field global to the image and just having one chose the earliest date it was seen, rather than needing it per-source. This data could probably be auto-fetched too when using the image fetch from some source URL feature.
With this people would be able to sort images by “original upload date” rather than when they were uploaded to Derpi or search on such a range, which could help organize things chronologically better (for those who want to search the site like that). It’d also be useful just to see what timeperiod a piece of art was made in if in the future the original source is dead and the source page wasn’t archived anywhere.
I’d imagine the sorting/searching would use the originally posted date if available otherwise just fall back to the upload date, or something like that (since obviously most stuff is not going to have this metadata until people start adding it to existing uploads).