Please check the K2 Parameters as that contains the Master image sizes. This can be found in the K2 backend when you click one of the K2 functions, like item, category. It will be displayed in the top right of the screen.
Additionally, the K2 Category view has parameters that gives the ability to over ride the master settings allowing you to create custom image sizes for that category.
Once you have added an image to an item and saved it, then wish to change the size, you will need to remove it first from the item and then reattach it.
Let us know how you get on.
SimonK2 Support