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 two images above show what happens when an image optimized for dark or pale backgrounds is put on a different coloured background. You tend to get a pixilated line around the transparent image. If you want any particular image changed in colour or background colour, specially for your web page, email me.