Site: http://web.missouri.edu/~mdmnq5/vrg/
Atahualpa 3.6.4 with bugfixes 1-6
Wordpress 3.1.1
Tested in Chrome 11 Beta and Firefox 4 with Firebug.

On the front page of my site I have a picture which refuses to show at full size (500px width). The image has been set to full-size and center in the "Insert Media" dialog. The resulting HTML code is:

[caption id="attachment_6" align="aligncenter" width="500" caption="From left to right: Dr. Stefano Pittalis, Dr. Oleg Shalaev, Dr. Giovanni Vignale, Matthew Mower, Tianyu Liu"]<a href="http://web.missouri.edu/~mdmnq5/vrg/wp-content/uploads/2011/03/group.jpg"><img class="size-full wp-image-6" title="Group Photo 2010" src="http://web.missouri.edu/~mdmnq5/vrg/wp-content/uploads/2011/03/group.jpg" alt="Group Photo" width="500" height="375" /></a>[/caption]
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The width and height are correct in the <img> tag. This screenshot of Chromes's "inspect element" feature suggests the div container holding the image is plenty wide to accommodate its full size (the td#middle tag alone has a width of 608px).

Relevant theme options:
Style & configure LAYOUT
Min/Max Width: 840px; (fixed width layout)

Style IMAGES
Images in posts: Caption style:

text-align: center; margin: 0; padding: 0;

Caption Text: Style:

text-align: center;
font-size: 12px;
padding: 2px 4px 2px;
margin: 0;
color: black;

I never noticed that small difference before. I am not sure why but the theme is set to show the full size images at 96%. To override that you could add the following to ATO>Add HTML/CSS Inserts>CSS Inserts

.post img.size-full {
    max-width: 100% !important;
}

Thank you for the solution, it works.