Can I create a Windows VirtualBox image on a Linux computer and then use it on my OS X Mac? I want to try Windows 7 under VirtualBox on my Macbook Air, but I have no optical drive. Rather than going out and buying one, can bypass the optical drive installation by making a Windows 7 VirtualBox image on my Linux tower and then copy that image over to my Mac via rsync? 

And are there any VirtualBox configuration settings I may need to use to accomplish what I have in mind?
Yes, you can easily copy virtual machines between systems.  

The easiest way would be to just create the virtual machine on your linux machine, and when you are ready to copy it over, you can export it as an appliance.  This will give you a single file to copy over to you Macbook Air.

Once you have the file copied over to your Air, you can then import it there, and you will be all set to try it out.