http://www.xnview.com/
May allow you to open your images and save them in a format which is supported under paint. If looking for a good editor, try the freeware editor The Gimp
As for the reason why it does not open, I am not sure. I never did open a jpeg in MS Paint, not believed it would work. As MS Paint is a bitmap editor. Are you sure you did not have another program which you used to open them or that they were saved as a bmp?
It's possible, if MS Paint can handle jpegs, that the ones your trying to open could be a progressive jpeg format.
Searching the board will reveal many good graphics apps, many freeware versions and shareware version of several popular programs.