The images are dual licensed under the Creative Commons Attribution-ShareAlike license (boilerplate here) and the GNU Public license, version 2 or any later version (boilerplate here).
Licensing images udner the GPL is not that common, and sometimes confusion arise about what the preferred form of modification is in this case. This is intended to clarify my interpretation.
The major reason for dual licensing the images is because the GPL and the creative commons licenses don't play well together. While I doubt anyone will use these images in a GPL'd work, I see no reason to make this particularly hard.
I've always felt this is fairly self-evident. This is the copy you would start with when making further modifications. For my purposes, this is the Original JPG image captured by the camera, or, when it exists, the original CCD-RAW image. For the scanned images, it's the PNG scan I saved.
In cases where the original is not clear, I've tried to indicate it on the web-page, but in most cases, it should be obvious (it's the highest resolution image). When the original differs from the full size image in the gallery (because I've tweaked the image, or because it a CCD-RAW image) it should be clearly linked from the gallery image.
What this means, is, if you modify the image and distribute it, you should provide as the preffered form of modification the version you would use to make further modifications. If you've changed the image to a layerd gimp file so you can easily tweak stuff on specific layers, then the layer version is what is required. If you always go back to the original JPEG image, distribute that, and so forth.
And no, I see no reason why the conditions of either license should be considered onerous. If they are, find some other source for the images you need.