In general disk images are big and uploading via browser isn't reliable considering a disk image having several hundred GB or more.
If you are talking about iso images, than you just have to enable it in the storage configuration.
Coming from another proxmox host, you could use a backup to restore your vm, just as an example.
If your storage supports the upload of iso images you would click on the content field in your screenshot and select iso images. LVM is a block storage this means you can't just upload a iso image to it.
Please check out our documentation to get familiar with the storage concept: https://pve.proxmox.com/pve-docs/pve-admin-guide.html#chapter_storage
Thanks - read it, but found it VERY difficult reading.
A 'simple' example of how to add a disk to an existing set-up would help.
I followed a blog post, but apparently it stopped short of explaining the bit I am trying to achieve.
Thanks for your interest in my problem.