Get a broken image replacement, such as this placeholder and save it to your webserver (available on the same domain).
Add the following entry to Apache (virtual host or .htaccess file):
<IfModule mod_rewrite.c>
RewriteEngine on
RewriteCond %{REQUEST_URI} sites/default/files
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^ default.png [L]
</IfModule>
For Drupal the recommendation is to sites/default/files/.htaccess to not to hack core.