there are a total of 3 unlockable character CGs in the game. 4 if we include Walter. 2 are from Lars while 1 is from Anders. You will get Lars 1st CG in Day 7, while his 2nd CG is in his episode. Anders 1st CG is unlockable in his route at the first time you look at his sketchbook. The rest of the character CGs besides Walter aren't unlockable in the public build and is available at Day 7 probably.