It is like Adjust said. Definition in the CSS is not enough, you have to use it. Try the following:
Code:
<p>bla bla bla</p>
<div class="Center topbottom"><img src="pic1.png" style="width:339px;height:288px;" alt="Pic 1" /></div>
<p>bla bla bla</p>
or
Code:
<p>bla bla bla</p>
<p class="Center topbottom"><img src="pic1.png" style="width:339px;height:288px;" alt="Pic 1" /></p>
<p>bla bla bla</p>
which will probably work better.