Install pypng: pip3 install pypng
python3 png2ass.py picture.png --start-time 0:00:10.00 --with-ass-header --text-prefix "{\fad(500,500)}" > nekopoi.ass
--layer: the layer of this picture (0 by default)
--start-time: the beginning time of the displaying png (for eg 0:00:10.00)
--end-time: the ending time of the displaying png (for eg 0:00:20.00 - 1hour by default)
--pos: the position of the png, by default "0,0"
--text-prefix
--text-suffix
--with-ass-header