If a player's photo cannot be changed then it is most likely due to the fact they are already signed on with another league and the photo is locked. .
This is a common issue where a player plays for 2+ teams in different leagues.
Once photos have been uploaded they are then locked for the season.
In order to upload new photo - you will need to contact your County FA to have these photos unlocked.
This will then unable you to upload a new photo to that player.