How can I simply and quickly upload just 1 photo from my iPhone to a PC or Mac?
You can email it as an attachment to another email address. For emailing single photos, I got around this problem by setting up a secondary email account and emailing it to myself there. Then I checked the email account from my laptop. For multiple pictures, I had to use a windows machine (Windows 7). I just plugged the iphone into the PC's USB and it worked, reading it as a data drive.

It may also work to save it as an attachment in a draft message. Then synch accounts and look in the drafts folder of your computer.