Images, JavaScript, and Flash files that PHP sends to the browser are not affected by the GPL because they are data. However, Drupal's JavaScript, including the copy of jQuery that is included with Drupal, is itself under the GPL as well, so any Javascript that interacts with Drupal's JavaScript in the browser must also be under the GPL or a GPL compatible license.
L. Hong, E. Chi, R. Budiu, P. Pirolli, and L. Nelson. Proceedings of the Working Conference on Advanced Visual Interfaces (AVI '08), Napoli, Italy, page 65-72. (2008)
G. Murphy, and C. Ryan. GECCO '07: Proceedings of the 9th annual conference on
Genetic and evolutionary computation, 2, page 1755--1755. London, ACM Press, (7-11 July 2007)