I'll try and explain what he means.
Each team with player pictures has a config (XML)file that tells the game what picture to display for each player. The players with disappearing pictures are players that are on loan at the start of the game. They have a config file for the team that owns them and the team they are on loan with. This confuses the game and it doesn't know wether to display the picture added to the game.
It can be fixed though...
I'll give you an example: I downloaded a Fiorentina facepack as I signed Portillo who was on loan from Real Madrid. He has a entry in the real Madrid config file but there is no picture so none displays. When you download the Fiorentina facepack there is a config file for that with an entry for Portillo. So the picture doesn't allways display due to the confusion.
To fix it simply open up the config XML file of the team that owned that player in notepad and remove the line that refers to that player and save it. The picture for the player downloaded should then display.
I hope that makes sense, I could have explained it better, but I'm not an expert on Graphics and PC's generally.