Simplemente conectarse al servidor de Proxmox por medio de SSH y descargar la imagen con wget/scp u otra herramienta en el direcorio /var/lib/vz/template/iso.
Por ejemplo para descarger el ISO de OpenBSD 5.5 de 64 bit
vm:~ # cd /var/lib/vz/template/iso vm:/var/lib/vz/template/iso # wget http://mirrors.nycbug.org/pub/OpenBSD/5.5/amd64/install55.iso -O OpenBSDamd64.iso --2014-09-16 18:54:16-- http://mirrors.nycbug.org/pub/OpenBSD/5.5/amd64/install55.iso Resolving mirrors.nycbug.org (mirrors.nycbug.org)... 66.111.2.14 Connecting to mirrors.nycbug.org (mirrors.nycbug.org)|66.111.2.14|:80... connected. HTTP request sent, awaiting response... 200 OK Length: 248283136 (237M) [application/x-iso9660-image] Saving to: `OpenBSDamd64.iso' 100%[=============================================================================================>] 248,283,136 8.03M/s in 35s 2014-09-16 18:54:52 (6.86 MB/s) - `OpenBSDamd64.iso' saved [248283136/248283136] vm:/var/lib/vz/template/iso # ls CentOS-6.5-x86_64-bin-DVD1.iso debian-7.6.0-amd64-DVD-1.iso linuxmint-17-xfce-dvd-64bit.iso ubuntu-9.10-server-i386.iso CentOS-6.5-x86_64-bin-DVD2.iso debian-7.6.0-amd64-DVD-2.iso OpenBSDamd64.iso ws2k3-ent-en-r2-cd1.iso CentOS-6.5-x86_64-LiveDVD.iso debian-7.6.0-amd64-DVD-3.iso SLES11SP1.iso ws2k3-ent-en-r2-cd2.iso