The images vary in numbers of frames, detail and file size. Animation that use more frames, colours and details are larger, so if you want a small file size you will not be able to use the newer higher resolution, more detailed images. In general the older the image (and some were made nearly 10 years ago..) the lower the resolution and number of frames. This is because they were designed with the constraints of the internet 10 years ago before high bandwidth access was so readily available.
The images shown on white have been optimized for a pale background while the ones on the black background have been optimized for dark backgrounds. If you want any particular image changed in colour or background email me.