I am using the K2 Multiple Images plugin on several pages.
Have not you thought about adding images crop and automatic thumbnail creation?
Two years ago, I wrote to you about adding this feature in the Joomla Article Images Upload and Gallery plugin. It came out very well
Your Joomla! and K2 extensions are very good! Thank you!
This plugin designed to use standard functions which K2 already has. You can configure image crop size in K2 generic parameters section Images. You can use XSmall image size as a thumbnail. For disable some images types (e.g. Small, Medium), you can just use zero width.