modify 0.2 to 0.18 in line 308 in image.py
modify 0.2 to 0.18 in line 308 in image.py

NetEase/airtest,wenrr89/airtest,NetEase/airtest,wenrr89/airtest,wenrr89/airtest,NetEase/airtest,NetEase/airtest,wenrr89/airtest,NetEase/airtest