The look and feel of your website must reflect your over all vision for your company or business. In my opinion, the "Header" is a very important aspect in creating the over all look. If the client has a logo, that is always taken into consideration, and a priority factor. Then the task of finding just the right visuals - image - to incorpoate into the over all feel. "Every picture tells a story," "a picture is worth a thousand words," you know the sayings, and it's true! I usually pull the entire color theme from a single picture, perfectly coordinating it through out the design.
It would be nice IF we could use beautiful high resolution images all over the website! BUT, this would not be wise as the pages would become to heavy, making the site useless. Always optimize your photos to the best download size while retaining the optimal quality... easier said than done sometimes. One or two fantastic images is always better than many images of less quality and impact.
It is always helpful IF the client has an "image" in mind or available. While working on http://www.wisernow.com/, my client had an image of an apple. That was all we needed, and the entire look took off beautifully!
Keep in mind that knowing WHAT you desire, and being prepared from day one in providing examples will only result in a more satisfying web experience.
In my opinion, the entire web page should not exceed 50k at most.
