Promox has built-in support for installing templates, but they’re pretty outdated and not well-maintained. openvz.org, on the other hand, regularly updates their templates, and has a wide selection to choose from.
In order to make these templates available for use in Proxmox, it’s simple. Get the links for the templates you’d like from the link above, and download them like:
wget http://download.openvz.org/template/precreated/ubuntu-14.04-x86_64-minimal.tar.gz \
All of the links were placed together with one wget command, but you could also do a wget for each template.
You will now see the templates in your local storage. Note that if you use alternate storage (e.g. GlusterFS), you’ll need to update the template path above accordingly.