After I run helm uninstall I was expecting that it will remove all resources related to specific release, but images pulled on helm install are still there.
Is this expected behaviour and is there a easy way (flag perhaps) to remove images also?
It is expected. I believe image cache is handled on CRI level and helm / kubectl have no control over that, so there's no flag.
You can use projects like docker-gc to set up a cleaning schedule thru helm (running a GC daemonset), or rely on kubelet's built-in GC controlled via kubelet process flags.