I am curious to know about the image sizes you are working with here.
What is the resolution//pixel width and height of the largest image? What is the file size in kb?
Are these color/photorealistic images? Line drawings? Black and white?
There are some things that can be done to optimize the image size/file size, but different types of image have different requirements.
16-million color (24bpp) PNG gives very good results for almost any image, but the file size can be really huge.
Line art can be saved as 8bit 256-color/greyscale png for much smaller files, they don't need 16-million color. (You can even use "palleted" image, reducing the number of colors to as low as 32, and then save as a 256-color png -- fewer colors will mean a smaller image. Don't go as low as 16-colors though --- some older Sony readers, and perhaps others, cannot render 16-colors correctly and display an awful mess. Also avoid the "Web-safe" pallet, it gives truly ugly results.
256-color Png cannot render photos and gradients well, you will see "banding" or what is sometimes called the "posterized" effect. Those kind of images can be saved as jpeg, with the least compression you can afford to keep the compression artifacts to a minimum, and still get a smaller file size. Compressed jpeg almost always gives a smaller file-size than png, but also a lower quality.
If you down-size image resolution, you might check out a very simple little utility called Color Quantizer -- it offers a range of different algorithms for resizing, and with experimentation you can get really nice-looking results. (You DO NOT want to use "Pixel Resizing" --- it gives the worst results in the world.)
http://x128.ho.ua/color-quantizer.html ---- site is in Russian, thank heaven for Google Translate!, but the application GUI is in English.