There are a lot of problems with measuring pd accurately with a photo. Ideally you need to ensure the pupils are aligned (i.e. the head can't be turned) which i

There are a lot of problems with measuring pd accurately with a photo. Ideally you need to ensure the pupils are aligned (i.e. the head can't be turned) which is really hard to tell from a photo (there's no depth). The credit card trick somewhat works since you can guage depth given the size and positioning of the card, but the errors still can end up in the 2-3mm range fairly easily. The best mechanism would be to create something like a Google cardboard to guarantee the head is properly positioned and then perform the measurements.