Once a graphic or icon has been loaded, it can be re-used without slowing access down. Therefore, re-use as many graphics as possible, rather than having new ones on every page. Use text, small icons, or thumbnails as stand-ins for larger graphics. To speed downloading of your page, all of your major images should be small in-line thumbnails, icons, or even just text which permit the visitor to download the large graphic if they want to. If your images are large, indicate their size so that readers can decide if they have enough bandwidth to warrant downloading the image.
Thumbnails are smaller versions of a larger image.
Many image processing programs have the ability to save an image in a reduced size that takes up much less memory. This in effects lets you create your own custom icons to give the reader a smaller version of what you want to show them. Use captions Pictures in other media almost always have captions. Use descriptive captions with every picture.