highlight the image then go layout paragraph then select center.
this is the wrong way, because you'll get some more vertical space between image and caption which does not look fine. This space comes from the environment center. Use instead as first line in the float \centering in TeX mode.
I belive you can automaticly centre all images but putting something in the preamble but cant remember what it is now. Have a bit of a look in the past postings
http://www.educat.hu-berlin.de/~voss/LaTeX/floats/floats.phtml#center
Herbert
