Save it as a .jpg and when asked, move the slider to down until you want but the only downside if that it'll look choppy and off color. :\
Go to, Save for Web, choose JPEG and set the quality to 80 / very high. It'll leave the quality the same and drastically reduce the size.
.PNG uses dynamic color tables so it is the format that takes up the least amount of disk space. Limit the number of colors in your image and that will gretly reduce the files size. Some times you can barely tell the difference, even if you take a out a few hundred colors.
I got Adobe Photoshop CS2 and when i save a file then it says file compressed its good then i dont have to do it later.