The GPL FAQ on template licensing.
Within the “Removal Of Over 200 Themes?” thread inside the wp-hacker mailing list, Amy Stephen pointed out about this part of the GPL FAQ regarding template licensing:
Previously, WordPress veteran andrea_r tweeted:
Can graphics be GPL? Why or why not? I say not.
According to the FAQ, she is correct. IANAL, but it looks like the solution for those who wanted to sell themes is to GPL-ize the theme’s PHP files and sell the CSS/JS/images package instead.