So I can state that there hasn`t been a discussion about a special GNU licence for pictures yet and nobody dissagreed that there is probably a need for one.
I don't think that the question of a "license for images" is a well-posed question. The ethical requirements depend more on the purpose of the work (entertainment or education, say) than on the medium (image or text, say). Images in textbooks should be free, including the freedom to publish modified versions, and the GFDL is a good license to use.
Images meant for entertainment are a different issue, and I would probably recommend using one of the Creative Commons licenses for them.
There is no difficulty in including images in a GFDL-covered work. It isn't necessary to mention images specifically in the license because the same conditions apply to text, images, and whatever else the work may contain.