Originally I followed this guide but I ran into issues with the host banner which displayed html code rather than the image that it pointed to.
In the above screen you can see my customized host and MOTD banners along with the names of the files that control which images you see - host.txt for the host banner and motd.txt for the MOTD banner. These files are located in the /hlds/l4d/left4dead directory on your server.
Now the images that you select will be resized depending on the screen resolution of the players connecting to your server. So I made my images the following sizes (which should support a resolution of 1920 x 1080:
host banner - 605 x 121
MOTD banner - 1076 x 538
So I created both of my banners as .jpg files and uploaded them to photobucket.
Next we need to look at the html template that we need that will point to the images that we uploaded.
<head><title>Left 4 Dead 411</title></head>
<img src="URL for host or banner image" style="width:100%;height:100%;" />
Copy and paste this into two new files named host.html and motd.html - just make sure that you change the "URL for host or banner image" part according to your URLs for the host or banner images on photobucket.
Upload host.html and motd.html to some webspace that allows public access - I used Sitesled to host mine for free!
On the server you just need to edit host.txt to contain the URL to your hosted host.html file and then edit motd.txt to point to your motd.html URL.
Now in theory you should be able to put the contents of the .html files that we created directly into the host.txt and motd.txt files on your server and have it work - but I found that (for whatever reason) the host banner simply would not work - it displayed html code instead of the banner image. That is why I hosted mine as .html files - and once I did that everything worked.
The only additional issue that I had is that the motd image seems to be resized smaller and then streched slightly (rather than being viewed at its native size) at higher reolsutions - so my advice (if you experience this issue) is to keep your text fairly large so that your MOTD does not look bad.
So here you can see what the host banner looks like in game by pressing the Tab key.