I have teams that are doing surveys and taking photos of respondents (in and around their homes, often outside) with permission to take and use these photos for facial recognition. We're complying with a wide range of privacy laws, disclosures, etc.

Now, I have to give these teams instructions for what photos will work best with facial recognition tools. What would the best set of instructions be?

Thus far I have:

 - Take photos straight on (rather than at an angle)
 - Avoid shadows across faces
 - Ensure there are no other people in the picture
 - Ensure there are no pictures or other 'faces' in the background

Anything else you'd recommend? Are these instructions good? Does filesize / resolution tend to matter a lot? What about lighting? Thanks.