For reference, I've posted examples of how different images appear using JPEG and PNG formats under different settings. For large scale animated sprites, such as rotating planets, it will be useful to have smaller files to reduce download sizes. Although people will always complain about download sizes and many other things; bandwidth costs money; times each download by the size of the game each month; we can at least help reduce the download size by using 256 color PNG or compressed JPEG images for large animations. Although 256 color PNGs obviously don't look good on images featuring many colors. We already know that there are different compression settings for images, just like with mp3 and ogg for music, but the images below reveal the results.
200x200 pixel image: Now obviously in the real case scenario, I am dealing with animations larger than 200 pixels, i've used this size for quick page loading. You can click the images to see the proper result. 50 fps is the target speed as discussed, not all kinds of animations will need 50 frames per second, it depends how entities animate, most have been exported at 20-30 fps; Even 1 fps for things like regular clocks.
1: 24bit PNG - 56kb (Internet Explorer 6 users might not see this correctly)
2: 256 color PNG - 24 kb
3. Jpeg 100% - 37 kb
4. Jpeg 50% - 11kb