There are a couple of ways to do this. It looks like the DNS for your domain is being managed by GoDaddy and so if you wanted to modify your DNS, you'd have to log into GoDaddy's control panel to do so.
First thing, decide on your new subdomain like "blog.rlewisphotos.com". Then you have to sign up for hosting using blog.rlewisphotos.com as your domain. Once this is done, your host will be able to give you the IP address of your new account which you will need in the next step. (not familiar with zenfolio enough to be able to tell you if they can host your blog too in which case zenfolio would tell you the IP address to use)
Now you have to update the DNS information for your domain. You would have to log into GoDaddy's DNS management system and add a CNAME record for your new subdomain. So for example, if you wanted to add "blog.rlewisphotos.com", then you would need to add alias "blog" as a CNAME record that points to the IP address provided in the previous step. GoDaddy has a how-to article on this:
Once the DNS change propagates globally, when you access http://blog.rlewisphotos.com, it should be pointed to your non-zenfolio hosting account.