You need to edit your pics in a image editor almost all have a resize feature. Mesure your image sizes in pixels at 72dpi keep in mind that if your site is designed to be displayed on a screen with a 800x600 resalution, a 800x600 pixel image at 72 dpi will fill the screen and ajust the images down over to suit.
To save time on uplading and downloading for your site viewers save graphics as GIF images and photos as Jpegs with a good amount of compression.
If you intend to use a lot of large pictures on your site use small images (thumnails) that link to pages with large images. This will allow site viewers to view your site with reasonable speed and also give them the opertunity to view your images in all there glory.
If you only intend to use your scaned images for web use may I sugest scaning them at the required sizes at 72 dpi someone at your libery should be able to show you how to change the settings if you are not sure