quickest way to transfer iPhone photo to "/tmp"
From your comment, I will ignore the "accepted" methods, such as Photo Stream.

Your question is still somewhat generic, as "quickest" can mean different things to different people in different situations.


If you are physically near your Mac, AirDrop. Here is an apple support document explaining the procedure.
With a USB cable, connect your iphone to your Mac and open the Image Capture application. Depending on your settings and OS versions, you may need to grant permission on the device. You can select one or many photos, and choose import, or just drag them into a folder. There is an option to delete them from the phone after transfer as well.
Without a USB cable, your method is actually what I would recommend, although with a twist. I attach the images to an email, then save it as a draft, retrieve the emails from the draft on my Mac, then delete the draft. This has the added benefit of not creating a copy of the email in your sent folder. It seems like this might be important to you as your concern with standard sharing methods is unknown "leakage."