You are probably using a CDN like CloudFlare. K2 item images' filenames don't change as they are tied to the item's ID.

What you need to do is enable the option "Force 'item modified date' as URL timestamp on image paths" in the "Images" tab of the K2 component settings.

If that option is enabled and you modify a K2 item to change its image, yet you don't see the change, the problem is most likely somewhere else.

If you have CloudFlare, make sure caching is set like this: jmp.sh/LLHy3zL

If "ignore query string" is what you have now, this would explain why you don't see any change...