That's a great idea. However, the following must be ensured:
* The mip-mapped images can be purged manually.
* The mip-mapped media files are removed if a software change affects
how images are rendered, (e.g. if new metadata are considered)
* Mip-mapped media files are updated if an image was overwritten; it is
essentially important that *all* of them are updated. Inconsistencies
here would cause a lot of confusion.
* Currently, JPEG images are sharpened. Does this affect how the
end-result looks like if they are sharpened twice? Do you expect other
quality-issues, for example for GIF or PNG? Is there a comparison of
applying down-scaling twice with doing it just once?
--Rillke