Get the new Version by Joomlaworks: https://github.com/getk2/rebuildK2imageCache
Rebuilds the image cache directory for the Joomla Component K2
It can take some time to rebuild the image cache, on my MacBook Pro it takes 5 seconds for rebuilding all sizes for one image.
How to use:
Copy this file as rebuild.php into "JPATH_ROOT/media/k2/items"
Check Variables, use a size of 0 for not processing this image size
run it "php -f rebuild.php" (it overwrites existing files without notice)