This is a chunk of code I've had around for a while... fairly useful, felt like sharing.
Basically you're looking in a given folder with <cfdirectory> and pulling out a random image by counting between 1 and the number of images found. Just enter the relative location of your images folder and you're all set.
<!--- set the relative path to the folder with your images --->
<cfset imagesFolder = "./">
<!--- code to look in directory, choose one image at random --->
<cfdirectory directory="#expandPath(imagesFolder)#" filter="*.jpg" name="getPics" action="list"/>
<cfset maxrows = getPics.recordCount/>
<cfset startRow = randRange(1, maxrows)/>
<!--- if one or more photos was found --->
<cfif maxrows gt 0>
<cfoutput query="getPics" startrow="#startRow#" maxrows="1">
<img src="#imagesFolder#/#name#" alt="#listFirst(name, ".")#"/>
<br /><br />
Hit refresh or reload to get a different image.
NOTE: you might want to give the image a different alt tag, like the name of your site, or... whatever. (this code will use the filename of the image). This same code can be adapted for any files in any directory - change the 'filter' attribute to allow whatever file extension you like.
With CF8, you could get also the height and width of the image to create proper height/width attributes for the <img> tag.
[Update 1/22/08] This mini-tutorial is now featured on learncf.com