If you are on SSL and still using SMF 1.1.x
It is relatively easy to backport the image proxy from SMF 2.0.x to SMF 1.1.x since it is standalone
For SMF 1.1.x
Copy the proxy.php from SMF 2.0.x to the root directory of your forum
Copy Sources/Class-CurlFetchWeb.php from SMF 2.0 to your SMF 1.1.x folder.
Create a cache folder in the root directory of your SMF and make it writable.
In your Settings.php
Add to it
$image_proxy_secret = 'yoursecrethashhere';
$image_proxy_maxsize = 5190;
$image_proxy_enabled = 1;
Lastly you need to make the edits less than 10 that say image_proxy_enabled or image_proxy_secret from the sources/load.php and sources/subs.php files in SMF 2.0 just refer to similar code in SMF 1.1.x