What I do is set the width of my page to no more than 900px. That way people with even old monitors won't have to scroll and I don't like fluid layout because it can mess with layout unless you are careful. Also fluid layout will cause some of your banner picture to be cut off if the person resizes too small. With that said in Photoshop I do the following.
- Open the picture I want to use but make sure it is at least as wide as the banner size
- Crop it to the desired dimensions. In my case I crop to 905 in x 165 in. I'm using "in" instead of "px" at this point and adding 5 to the desired size for a little overflow
- Tweak the picture as needed
- In Photoshop go to File>Save for web and devices
- Select the format you want to use (usually jpg - medium) and click Save
I would suggest doing all of this after saving your original picture in psd format.
I hope that helps as it is kind of hard to describe processes like this without show and tell.