Alternatively:
using PyCall
cv2 = pyimport("cv2")
img = cv2.imread("/home/g2-test/julia_Hands_on/cat3.jpg")
cv2.imshow("original image", img)
img2 = cv2.cvtColor(img, cv2.COLOR_BGR2GRAY)
cv2.imshow("gray scale image", img2)
cv2.waitKey(0)
cv2.destroyAllWindows()
should also work. With PyCall now supporting . syntax, a lot of python code just works in julia.