First: Sorry for my english. It's not my native language ;-)For K2 v2.3I think this option should be in next K2 version..
When you add an item in the Image tab you can add "Item Image". This
image is auto-generated in 'media/k2/items/cache' directoru in 6 sizes:

1. Generic (default)
2. XS (xsmall)
3. S (small)
4. M (medium)
5. L (large)
6. XL (Xlarge)

Usually you don't need all sizes, and you just use 2-3 sizes on your
site.
When you have many items with illustrations, 'cache' direcotory consume
more and more disc space.

By default K2 do not offer possibility of swiching off unused images
sizes.
Few changes in K2 files will give you that option.


In ZIP file you'll find all three changed files and tutorial_en.txt with full explanation how you can do this by yourself.

Thanks for your usable effort.. ;)

regards,
LN