I put the instruction about knowing the file storage location for when you back up the files in Step 7. In many cases the person doing the upgrade is not the person who originally installed Mattermost, or they don't recall the location that they specified, if different from the default.
I don't know why the profile pictures don't show up. Can you verify that the pictures are actually on disk at the correct location? On a default install, with
./data/ as the file storage location, the pictures are in
jeff@ubuntu64:/opt/mattermost$ sudo locate profile.png
You might have to do
sudo updatedb first to make sure the index used by the
locate command is current.
The strings of random-looking chars are user IDs.
If the profile pics are in the right place, check file permissions on the user ID directories.