Don't use irfanview to reduce the color depth if it actually uses more colors than that. Its dithering algorithm sucks and you'll get less out of png or zip compression. (photoshop does really well if you have it, but you'll want to recompress any pngs you get out of it)

If you press 'I' irfanview'll tell you how many unique colors there are in the image. With less than 256, reducing the color depth will help a lot.