Issue with images not appearing properly on a multisite installation

Hi there,

We've just taken over the administration of a website from someone else and are running into trouble with images loading correctly on one of the sub-directory sites. There are three sites:

(abbreviated domains):

ias.com/
ias.com/usa
iasusa.com/

Essentially there are posts on the domain iasusa.com which are mapped to ias.com/usa.

So if you visit this blog post:

http://www.iamstaggeredusa.com/article/how-much-should-guests-spend-on-a-gift/

It maps to:

http://www.iamstaggered.com/usa/featured/how-much-should-guests-spend-on-a-gift/

And the images on the second are not showing up correctly. If you inspect the element on these images, you'll see they are different:

The first is:
ias.com/assets/2012/03/wallet-moneyetc.jpg

The problematic second one is something like:
ias.com/usa/files/2012/03/wallet-moneyetc.jpg

.htaccess looks like this:

#<IfModule mod_rewrite.c>
#RewriteEngine On
#RewriteBase /
#RewriteRule ^wp-signup\.php(.*)$ wp-content/themes/arthemia-uk/wp-signup.php$1
#RewriteCond %{THE_REQUEST} /(wp-includes|wp-content)/.*\ HTTP/
# RewriteCond %{HTTP_REFERER} !^http://(www\.)?iamstaggered\.com/.*$ [NC]
#RewriteRule .* - [F]
#</IfModule>

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]

# uploaded files
RewriteRule ^([_0-9a-zA-Z-]+/)?files/(.+) wp-includes/ms-files.php?file=$2 [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?siteN/files/(.+) wp-content/blogs.dir/N/files/$2 [L]

# add a trailing slash to /wp-admin
RewriteRule ^([_0-9a-zA-Z-]+/)?wp-admin$ $1wp-admin/ [R=301,L]

RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(wp-(content|admin|includes).*) $2 [L]
RewriteRule ^([_0-9a-zA-Z-]+/)?(.*\.php)$ $2 [L]
RewriteRule . index.php [L]
# END WordPress

Any help would be very much appreciated -- thanks very much.