Centreing issues?

Add to your css:

#myGallery_23{margin: 0 auto;}

Create a div with a unique ID for it. How are you inserting it?

From the post editor one might see:

<div id="myunique">[slideshow]</div>

All your galleries will appear to have the class: galleryview. You might try my code on that class and see how she goes.