After having installed proxmox, you immediately want to go to the web panel and start creating some KVM guests. Unfortunately, there are no ISO files to create the guests from by default.
However, you can just follow these easy steps to download them.
SSH into your proxmox server.
Navigate to the relevant folder where we should store our ISO files
Use wget to download the relevant ISO file. The example below will download the Ubuntu 18.04 server ISO.