I'm now using Prince 8.1 with a free license for non-commercial. And I found that the pictures in the output are a little bit larger than the ouput from Print 7.1.

Does it have any css to config?

Are you specifying a width for the images, or relying on the default image size? You can use the prince-image-resolution property to specify an explicit DPI value for the image if you like.

I used the same html source with the same css on both Prince 7.1 and 8.1, but the outputs are not the same.

Yes, the behaviour has changed. Prince 8.1 does not use the intrinsic image resolution by default, you need to explicitly specify "prince-image-resolution: auto" if you want the old behaviour.

I've already tired to use prince-image-resolution with auto and normal, but the output is still larger. Any solution else?

You have to check what properties you are applying to the <img> element, eg. the width or min-width properties. Alternatively, if it is a background image you can use background-image-resolution. Without more information it is difficult to know what is going on.