Usa image_xscale o image_yscale con un valor de -1 para invertir el sprite ya sea horizontal o verticalmente
http://docs.yoyogames.com/source/dadiospice/002_reference/objects%20and%20instances/instances/instance%20properties/image_xscale.html
http://docs.yoyogames.com/source/dadiospice/002_reference/objects%20and%20instances/instances/instance%20properties/image_xscale.html