"Normally, there is no problem, if we load two images in jpg formats,... But ... we need to insert png image."
The original format of the image does not matter at all for whatever it is you're doing in matlab. Whatever format it was, you get pixel intensities in matlab.
Now, it may be that your original code assumed that the images were RGB images of a certain class (probably uint8 MxNx3) and that you're now getting images of a different type (greyscale or indexed) or different class (uint16?) but that is unrelated to the storage format.
So, we need more detail about the actual images once loaded in matlab (size and class).
