Copy from remote machine to local machine securely without logging

scp username@remote_pc_ip:/path_to_remote_file path_to_local_file

How to extract files from android system.img

To extract files from android system.img on linux PC, use below steps
1) simg2img system.img system_ext4.img
2) mkdir temp
3) sudo mount -o rw,loop system_ext4.img temp
Now you can browse or copy files from temp folder.

Get google android phone factory images

https://developers.google.com/android/nexus/images