RB, the most basic way to install the photo packs:
1) Copy the batter photos into the "../Players/Batters/" folder
2) Copy the pitcher photos into the "../Players/Pitchers/" folder
3) the piece I think you might be missing, cut and paste the contents of the "alias.txt" file that was provided with the photo packs that you downloaded into the "alias.txt" file in the ../Players/" folder.
Without the alias data from the download, the photo's won't show up. Basically the alias file lists the player, if it's a hitter or batter, the team name, year, and photo file name. It points the player to the photo.
My recommendation would be to register at Strat Fan Forum and bookmark the thread below, it contains many years of photo packs and alias files.
http://www.stratfanforum.com/sffipb/forum/57-player-photos/