Just create a Bookmark for your VM and it will automatically be imported at launch time (see Adding Templates).
We currently support the qcow2 and VMDK file formats (please contact support if you need conversion for another format). The container file is classified by its content, not by the file extension. For example
will both work as long the URL is correct.
The disk size in the template must be equal or greater than the file size of the image (the virtual file size). To utilize the extra space just add partition(s) once the virtual machine is up and running.