How to transfer ONE photo FROM PC to iPhone 6 From time-to-time I want to transfer only one or a very few photos FROM my PC (Windows 7) to my iPhone (currently an iPhone 6). 

I can find plenty of advice on how to transfer from the iPhone to the PC, but there seems to be a dearth (severe lack) of info on how to do the converse.

I have iCloud, Wifi, iTunes, and a cable. I (and I'm sure another of 10K of your best friends) will be grateful for instructions that are "written for grandma." (I'm not a grandma, but I am a late adopter.)
The Apple way is to pick a folder on your PC for sync. You do this through iTunes, by connecting your phone and picking the photos tab. Full information here: http://support.apple.com/kb/ht4236

However, if it's just 1 or 2 photos, the easiest way is to use Dropbox (you have Dropbox right?). Put the photo in your Dropbox, go to the same folder using the Dropbox app on your phone, click on the photo, click the little share button in bottom left, and then choose "Save Image."