I recently purchased and transferred my site to a new domain. It was updated to rachelsydow.com/picturethisbyrach. But I want my zen photo gallery at gallery.rachelsydow.com. I'm not sure what steps I need to take to move it without messing everything up.

I couldn't find a lot of information on this, if anyone could let me know what I need to do to get the final site relocated, that would be greatly appreciated!

Setup your sub domain to point to that folder and then re-run the setup script from that domain. That's all. Just in case, you cannot use the folder anymore then.

Sorry, I'm not quite sure what you mean by that. You could be a little more specific on what I need to do? Thanks!

Sorry, I meant COULD you! THanks!

If you want to use a specific (sub)domain you need your server to tell where to point to. By default it is the root of your webspace. But you want to point to a subfolder. You need to set that on your server. Ask your host how to do that.

Then you need to run the setup script of Zenphoto so it knows it is now in the root of the domain and not in a sub folder.
http://www.zenphoto.org/news/installation-and-upgrading
http://www.zenphoto.org/news/how-to-move-my-installation-on-the-same-server-