The tip explains you need a specific class.

html code is, for example:
<figure class="pull-center"><img title="a title" src="images/joomla_black.gif" alt="" /><figcaption>Center chosen in Image float</figcaption></figure>

In Protostar, the css are:

figure {
    display: table;
}
figure.pull-center,
img.pull-center {
    margin-left: auto;
    margin-right: auto;
}
figcaption {
    display: table-caption;
    caption-side: bottom;
}

In short: the template needs to support it, otherwise it will not work. Not a core issue.

Ooook thanks for your answer :)