How to pick a good photo for a portrait
You want to be close enough to be able to see a bit of detail in the eyes. Even light is not mandatory but the photo needs to have enough light to be able to discern color changes and features. Lots of small things can be edited out so if there is something you don't want in your portrait then be sure to bring it up.
Some not so great examples
These photographs might not have enough light to see detail. They might be to far away to get a nice crop of the head or are obscured by other things like glare and light shafts, dogs, or people.