There are a lot of things that you are going to have to think about when you are designing a website, one of those is graphics. This is something that a lot of people don't pay as much attention to as they should when they are designing a site and it really hurts the quality. It is important to make sure that your graphics are designed by a professional who understands graphic design from the perspective of web development.
Graphic design is an important part of web development in large part because modern websites require graphics in order to keep your visitors interested. Over the years websites have become a lot more sophisticated and if you want to have a site that looks professional you need to make sure that have good graphics. That being said you can't just randomly put up a whole bunch of graphics they have to be relevant to your page and fit together in a coherent manner. Nothing makes your site look more amateurish than having a whole bunch of unrelated graphics on every page.
The biggest reason that you need to have a graphic designer on your site is for branding purposes. This is what makes your site memorable; you want something that people will remember and immediately connect with your site. This serves several purposes, it helps to bring visitors back, it separates you from the competition and it helps you to build trust. This is why all of the big companies have a logo, and why you need one for your site as well. This is true of all sites but especially sites that are in the business of making online sales.
One thing that you have to keep in mind when you are doing graphic design on your site is that you have to make sure that all of the pages on your site look similar. You don't want people to think that they have left your site at any point so all of the pages have to have a similar look. At the same time you don't want all of the pages to look the same or your visitor will get bored. You need to find that balance of making each page look a little bit different while at the same time making all the pages look like they go together.
It is also important that you make sure that the graphic design on your site doesn't overwhelm the content of site. This is another fairly common mistake that a lot of people make; they realize that they can add graphics to their site so they just stick them everywhere. This can make it hard to read the content on your site. Since the goal of the graphics is usually to get people to read the content this is a mistake that needs to be avoided if you are going to have a successful site.