Your issue sounds a lot like what i had a few months back. Going up a hill with added tension, the chain felt like it was slipping. i too thought it was my chain, and like you i did all of the measurements with a ruler, ( i even used a chain gauge tool), but they all indicated no chain stretch. I finally brought it in to the shop and they gave the derailleurs a slight adjustment. The problem wasn't chain slipping, it was chain moving down a gear. My problem went away immediately. Other than the "slippage" i experienced on hills, the gears shifted with no problem whatsoever, so there was no reason to think the gears were out of adjustment. But that was definitely the issue, because it didn't happen anymore after the adjustment.
Not saying this is your problem, and i agree with others, your chain looks a little stretched, but it's something worth looking at if you haven't already.