You can use imencode:
encode_param = [int(cv2.IMWRITE_JPEG_QUALITY), 90]
result, encimg = cv2.imencode('.jpg', img, encode_param)
(The default value for IMWRITE_JPEG_QUALITY is 95.)
You can decode it back with:
decimg = cv2.imdecode(encimg, 1)
Snippet from here