Looks like it's something on the Facebook side. You gallery links this image:
http://www.merelyafleshwound.com/conten ... te-023.jpg
But at least we can try something. First, try adding a title to this image and see if it changes anything. If this won't help, could you please try and edit iv-includes/templates/index.gallery.phtml.
Find this block of code:
<?php if ($this->firstImage): ?>
<?php if ((ivPath::canonizeRelative(substr($this->firstImage->thumbnail, 0, strlen($this->contentPath))) !== $this->contentPath)): ?>
<img src="<?php echo $this->url(array('c' => 'xml', 'a' => 'thumb', 'path' => $this->firstImage->getPrimary())) ?>" <?php echo htmlspecialchars($this->firstImage->title) ?>" />
<?php else: ?>
<img src="<?php echo htmlspecialchars($this->firstImage->thumbnail); ?>?<?php echo htmlspecialchars($this->firstImage->getThumbnailMTime()); ?>" alt="<?php echo htmlspecialchars($this->firstImage->title) ?>"/>
<?php endif; ?>
<?php endif; ?>
And wrap it in a link like this:
<a href="/content/slideshow/haute-route-023.jpg> ... </a>
My guess is maybe facebook will like either image with alt attribute or linked image better.