My style for images:
div.image {
text-align:center;
margin-bottom: 0.25em;
}
For captions:
div.caption {
margin-bottom: 1em;
text-align:center;
font-style:italic;
}
A typical image page:
<div class="image" id="disc">
<img alt="Sony Data Discman" src="../Images/discman.jpg" width="100%"/>
</div>
The "alt" is primarily for sight-impaired readers using a device with read-aloud feature. It is polite to say what's in an image.
I try to avoid captions, preferring to put the information in the preceding paragraph.