# Assuming your images are all in one location.
RewriteCond %{REQUEST_URI} !/images
RewriteRule .* http://www.blah.com [L,R]

or...

# This can filter based on file extensions.
RewriteCond %{REQUEST_URI} !\.(jpg|png|gif)$
RewriteRule .* http://www.blah.com [L,R]