Getting hired to shoot images for a website is pretty common now. A lot of small businesses decide to manage their own sites. This is not a big issue, as long as you have your template or site available before the shoot. A photographer really needs to know what size image needs to be in pixels before they shoot. This helps the photographer shoot each image in the correct ratio and avoids important information getting cropped. Not knowing image sizes or ratios can result in re-shoots and extra costs. It also benefits the client because I can size all images for the client and make them ready to use.
A quick note for clients. Color Management is very important for photography. It ensures color to be true to a viewer. Most photographers use the color space Adobe RGB or Adobe 1998. Any image used on the internet should be in sRGB. Make sure your photographer or webmaster converts files to the correct color space.
Below, I will post a new page I am devoting to Website Information. I show examples of how image ratios can affect your photograph.