I have a look on improvements of cache manager.
as far I can see, there is a new button "Cleanup image cache sizes".
what is the difference between the exsting one "Purge Image cache"?

Totally different.

• "Purge image cache" clears the cache
• "Cleanup image cache sizes" clears the sizes registered to the cacheManager. It works the following way:
  • "admin" sizes is always kept since always needed as are "default" sizes
  • Sizes of themes not available are removed (the list to select also filters out inactive themes as you most likey don't need them in that moment)
  • Sizes from inactive plugins are removed
  • All custom sizes - those you set manually on the plugin options - are kept since there is no secure way to know if they still may be needed or not.

ok that's clear