Everybody who knows a bit about GPL said it for years: WordPress Themes must be licensed GPLed or compatible, at least for the PHP and HTML Code. Matt made a request for clarification to the Software Freedom Law Center which did answer in detail.
While the answer is overall of clear duct, I do not go with the comment of CSS. It’s properly stated why the HTML should be licensed compatible to the GPL. But then that HTML is heavily interakting with the CSS inside the Browser afterwards. So for CSS this could be discussed with more details. Even thought the argument provided aren’t that bad.
For Pictures it’s clear and it’s well a statement has been made, that it is no problem to even license them under GPL as being part of the software.