When the image is displayed, the background will show through the portions of the image that are painted in the given color. This will make your icons look much better on a fancy background and can even add some 3-d look to your icons and images. Use the eyedropper to select one. GIF interlacing allows the image to be partially drawn while being transferred. This will allow your image to start out blurry and sharpen into focus.
Visitors can select the image or get an idea of it's contents before it finishes being transferred.
Almost all images look fine with 32 custom colors. We will address this more tonight. For photos, try reducing JPEG quality factor. Go for quality first. Limit the size of images.