Does it is very large for using 1.57 (average) kb for each sprite/image for displaying on the screen (may contain 3 or 4 sprite on the screen at one time)?
Thanks for your reply~
On my current project my images range from 500 bytes to 3.5kb. Used for backgrounds or as part of a sprite strip for an animation of an element in world space. I always use an adaptive palette and save out as 8 bit pngs (from fireworks or ImageReady). I then use png crush (command line compression app) to compact them a little more.
Pngs often consume quite a large of space for palette information (300 to 1000bytes) so it really helps if you can reduce the colour palette (i normally try and reduce the palette down to around 16 colours, sometimes 32 if the image is large and has a large variety of colours.
Thanks for your reply~ 
I use photoshop to draw it and save it for 8 color png, may be my sprite is large, it is about 57 x 64 for each sprite, but when I try to reduce the size, the image doesn’t look very well. 
Also, Could you tell me what is “png crush (command line compression app)”?
Thank you very much~
Thanks a lot~ 