It just not that simple... and that's the first thing people must accept.
Here are some guidelines you MUST consider with EACH image.
1) Useage: is this a bg image? can it be tiled?
2) Resolution: similar to above. Do you really need a 1200 x2400 px image? reduce your image size to the dimesions size possible. This may be beyond some people, but try not to severe the same image for everybody... i mean it would seem that a 500px X 500px image is reasonable, but for smart phone with a screen size of 480 x 360 , it's pretty big. If you have the capability to know the device used, take advantage of that!
3) composition.. for large areas of flat colors ( not to be confused with subtle gradients) use GIFs/PNG-8 for complex images JPG/PNG
4) Support Not really a big issue, but if for some reason you have to deal with ANCIENT IE versions, PNG may be out of the question
5) Transparency if you need transparency.. you need GIF/PNG
6) Palette and quality this literally will vary with each picture. With GIF you could edit the pallette manually; it does take a good eye to not over posterize , but it can be done. Adding an amount of blur to a JPEG before saving sometimes help. Obviously you can set the quality to some extremely low # , but beware that that will cause artifacts in your image
7) Color mode. If you have B/W photos .. you can save a few KB by making sure you are not on RGB mode. ( seems obvious, but you will be surprised how often it's overlooked)
8) With PNG-24, you can drop unused channels.
Tools for the steps above: Photoshop, Fireworks, GIMP and PNG crush.
Hope that helps.