You will need install package - install media is for USB installation during bootup. Install packages are for online upgrades/downgrades.
Just grab the image with enhanced automation which gives possibility to integrate with eg. VMware NSX (requires license) and similar.
Regarding upgrade; Please be advised that when you connect the two switches in a virtual chassis, only one of the configurations will survice - so port configuration from "slave" switch needs to be copied to master-switch in advance.
Information about configuring VC on QFX5100: https://www.juniper.net/documentation/en_US/junos/topics/task/configuration/virtual-chassis-qfx-series-cli.html