Complex sentence: Early experimentation eventually led (1960 onwards) to an interest in producing "electron paintings" that would emulate the form of an animated television picture, using large rastered images generated by a computer programme and then painted by hand.
Split into simpler sentences: Early experimentation eventually led (1960 onwards) to an interest in producing "electron paintings" that would emulate the form of an animated television picture. Large rastered images generated by a computer programme were then painted by hand.