Your preset is using alfaerie-many, which has $flip set to true and an entirely populated $flipped array. When it is black's turn, $rightsideup is set to false, and this triggers it to use the piece name from $flipped instead of $pieces when $flip is true and $flipped has an element with the provided array key. Although you have rewritten $pieces to use .png images, you have not rewritten $flipped. So, it is using the piece names already supplied in alfaerie-many, which are all .gif file names. If you set $flip to false, this may fix things, though I still don't see any point in you using alfaerie-many if you are internally defining a set using .png pieces.
Your preset is using alfaerie-many, which has $flip set to true and an entirely populated $flipped array. When it is black's turn, $rightsideup is set to false, and this triggers it to use the piece name from $flipped instead of $pieces when $flip is true and $flipped has an element with the provided array key. Although you have rewritten $pieces to use .png images, you have not rewritten $flipped. So, it is using the piece names already supplied in alfaerie-many, which are all .gif file names. If you set $flip to false, this may fix things, though I still don't see any point in you using alfaerie-many if you are internally defining a set using .png pieces.