I have a gallery of images for which I don't know the exact heights or widths of images beforhand, I just know that they are smaller than the div in which they are going to be contained.
By doing a combination of line-height settings on the container and using vertical-align:middle on the image element, I finally got it to work on FF 3.5, Safari 4.0 and IE7.0 using the following HTML markup and the following CSS.
The HTML Markup
The CSS
div.painting
{
float:left;
height:138px; /* fixed dimensions */
width: 138px;
border: solid 1px white;
background-color:#F5F5F5;
line-height:138px;
text-align:center;
}
div.painting a img
{
border:none;
vertical-align:middle;
}