RuntimeError using cv.SaveImage in openCV

I want to convert a jpg file to png, but when I run this code :

from opencv import _cv    
from opencv.highgui impor开发者_如何学Ct cvSaveImage, cvLoadImage

cvSaveImage("bet.jpg",cvLoadImage("bet.jpg")) 

if __name__ == '__main__':
    pass

It gives this error which I don't understand :

Traceback (most recent call last):
  File "convert.py", line 6, in <module>
    cvSaveImage("bet.jpg",cvLoadImage("bet.jpg")) 
  File "/usr/lib/pymodules/python2.6/opencv/highgui.py", line 183, in cvSaveImage
    return _highgui.cvSaveImage(*args)
RuntimeError:  openCV Error:
        Status=Null pointer
        function name=cvGetMat
        error message=NULL array pointer is passed
        file_name=cxarray.cpp
        line=2780

I have my picture with the same folder of source code and the name of the image is bet.jpg

Any idea ??

The best choice is pyopencv:

import pyopencv as cv

img = cv.imread('01.png')

cv.imshow('img-windows',img)
cv.waitKey(0)
cv.imwrite('01.png',img)

From Python CV documentation, the CV2 method for converting a jpeg to png is: Python: cv2.imwrite(filename, img[, params]) → retval

For my example:

import cv2
filename = 'pic.jpeg'
cam = cv2.VideoCapture(filename)
s, img = cam.read()
picName = 'pic.png'
cv2.imwrite(picName, img)

VideoCapture is nice and general, and works with videos, webcams and image files.

I solved the problem, the image I took randomly from the Google Images doesn't load. Maybe it's encrypted or something I don't know. I tried it with other images, and worked very well. So watch out while copying images : )