Since you have no optical drive the only option appears to be USB.
Read these instructions carefully.
From the manual:
You may be able to install Windows 7 with a USB flash drive that contains a Windows 7 ISO
image downloaded from Microsoft. To check whether you can do so, open Boot Camp
Assistant, click Continue, and see if the option “Create a Windows 7 install disk” is available.
To install Windows 7 with a USB flash drive, you need the following:
-An ISO image of Windows 7 Home Premium, Professional, or Ultimate downloaded
-A USB flash drive with 4 GB or more.
Boot Camp Assistant erases the flash drive, and copies to it the Windows 7 ISO
image and software drivers that let Windows work with your Mac hardware
Yes you download the .iso from Microsoft
FYI There is a community for the virtual machines like Parallels which might be a better place for support questions. I failed to note your interest in Parallels from your first post. My apologies.