TinyPortal

Quote
Leverage browser caching
Setting an expiry date or a maximum age in the HTTP headers for static resources instructs the browser to load previously downloaded resources from local disk rather than over the network.

I understand that many of the images should be taged to expire after some time.
Is this theme related?
Is it something that can be set from the theme at each <img> tag, from SMF with a php function or at server level like with an .htaccess file? 

Well, sort of..but that part is a bit more complex. Images supplied by the theme can be somewhat leveraged by setting the theme headers in the HEAD section(its not usually, which means its using the browser default)..but downloading an attached image is all done in Sources and the theme have no control over it.