[Plugin: W3 Total Cache] Specify cache validator for non-wordpress resources?

Installed and setup w3tc last night (switched from Quick Cache) and seem to be having good results; raised my google page speed from average of 80 to 85-87. I wanted to see what else I could do so I tested it in Pingdom (got 80/100) and now I’m going through the items I can fix.

Here’s one:

The following resources are missing a cache validator. Resources that do not specify a cache validator cannot be refreshed efficiently. Specify a Last-Modified or ETag header to enable cache validation for the following resources:

(two examples:)
http://i4.ytimg.com/vi/kijwc1DTuM0/hqdefault.jpg
http://w.sharethis.com/button/buttons.js

Is there a way to specify a cache-validator for these non-wordpress resources? Maybe in .htaccess?

http://ww.wp.xz.cn/extend/plugins/w3-total-cache/