Note: If you have another way of geotagging your photos, skip steps 2 and 3.
I've just discovered a cool app for geotagging photos. Click here to read how gps4cam works, and so you'll understand the rest of this step. It has a quirk that can show up while processing the images... if the QR code you photograph is too blurry, it won't read it... but it doesn't know it can't read it until it's logged all the photos, which can take several minutes. So copy the last 4-5 photos (including the QR code photo, which will be the last image) into the "test in" folder.
IMPORTANT: copy the photos, don't move them.
Open the gps4cam desktop app. Select "test in" as your Pictures Input Directory. Select "test out" as your Pictures Output Directory. Click Go. If it can read the code, it'll process the images in a few seconds. If not, then take a clearer photo of the QR code on your iPhone and replace the other one and test again. Repeat if necessary until it works. FWIW, once you figure out the quality of image it's looking for (it's pretty forgiving) you'll probably get it on the first try nearly every time. Once it works, delete the "test in" and "test out" folders.
The next step is to run GPS tagging.