You and your web designer need to work closely on this. It is the principal reason that graphics you may have had designed for other purposes such as printed brochures are frequently not usable in web applications. Other parts of the answer are more technical and you probably need to rely on your webmaster for them unless you have a digital graphics background. However, this is not so easy to do. We accumulate little logos and awards, icons of off-site links, and the like.
Buttons multiply as new links to our own new pages increase.
All of these inoffensive little graphics start to add up. Eventually we need to rethink the graphics content of the page. I recently chucked out a half-dozen little logos of membership in various webmaster societies from our home page. Finally, when you add new pages to your site, plan on re-using graphics from existing pages to the greatest extent possible. Once an image loads for one page it can be displayed on another without being downloaded again.