The primary way to reduce the file size of an image is by increasing the amount of compression. In most image editing applications this is done by changing your settings in the Save As dialog box after you select a compressed format such as GIF or JPEG. Adobe Photoshop uses a separate Save for Web menu option (File, Save for Web) that gives you a lot of control over how your file is saved as a GIF or JPEG.
If you choose JPEG, try adjusting Quality (0% to 100%) to see how low you can go without unduly compromising the quality of your image. If you choose GIF, the easiest way to lower the file size is to lower what is called color depth, which is accomplished by lowering the number in the Colors box. There is no need to use more colors in your palette than the original image uses. If, for example, the original image is straight black and white, 2 colors are all you will need.
Another quick way to reduce the file size is by cropping out unwanted parts of the picture. After cropping, you can change the image’s dimensions to go even smaller. Most image editing programs have a command for changing the pixel dimensions of an image. Look for a command called “Image Size,” “Resize,” or “Resample.” When you use this command you will be presented with a dialog box for entering the exact pixels you wish to use. When you resize your image, you want to make sure the “Constrain proportions” option is enabled. This option will prevent your image from being stretched or distorted when it is resized.
If you are using your images on the Web, you should shoot for a file size of 30 to 100 KB per image. Try not to exceed 100 KB per Web page for the total of all images.