Hi Tadas Narbutas, can you provide a link to your site. About the Registration Problem are you using the K2 User module ? and set up the configuration properly. I know from personal experience that, if you happen to use Community Builder for Registration it doesn't automatically add then to the K2 User List.
In regards to your Category View issue, When you change Image size it only applies the changes to any new image you upload, not existing ones. And if you create a Category Menu item and display child categories they will inherit their various Category view settings.