HTML doesn't have an element that allows to insert a figure with a caption.
It was once proposed (see HTML3),
but never made it into HTML4. Here is one way to simulate such a figure
element:
<div class="figure">
<p><img src="eiffel.jpg" width="136"
height="200" alt="Eiffel tower">
<p>Scale model of the
Eiffel tower in Parc Mini-France
</div>

Then in the style sheet you use the class "figure" to
format the figure the way you want. For example, to float the figure to the
right, in a space equal to 25% of the width of the surrounding paragraphs,
these rules will do the trick:
div.figure {
float: right;
width: 25%;
border: thin silver solid;
margin: 0.5em;
padding: 0.5em;
}
div.figure p {
text-align: center;
font-style: italic;
font-size: smaller;
text-indent: 0;
}

In fact, only the first two declarations (float and width) are essential,
the rest is just for decoration.