GdkPixbuf.Pixbuf.new_from_bytes
Currently only RGB images with 8 bits per sample are supported.
You need to filter your png files through say imagemagik to get just the raw RGB data.
A close example:
imagemagick convert png 16 bit to raw - Stack Overflow

If you want GTK to handle all the PNG headers and compression, you need to use say new_from_file