I want to insert images into K2 item. How can I make them auto resize itself when I use pc, tablet or smart phone?

I think this is an editor feature, which I am not sure if there is an editor in Joomla! that offers this functionality.

You can use media queries but this will change all images inside K2 items.

Finally if you use a large image then the image can stretch end to end and it will not break in smaller screens.