I personally get all my iPhone docks on Devianart. There's a lot of good ones on there, and ask people in the thread as well, I've got a lot of help that way.
As for the image used for the dock, navigate in iFile or any SSH method to the themes folder. (var>stash>themes.xxxxxx) create a directory and name it. (Doesn't matter what, this is what will show up in Winterboard.) inside that create one named Bundles (case sensitive). Inside Bundles create another and name it com.apple.springboard and drop your image inside here. (Pull the image from var>mobile>media>DCIM>APPLE100/101, etc. if you just save it, it will be the very last one, its in order of your camera roll.) drop the dock in com.apple.springboard and rename it
SBDockBG@2x.png
If you want them to reflect more than they already do, you can find a dock theme like Mirror Dock and pull the png from there named
SBDockMask@2x.png and drop it into your com.apple.springboard folder.
Theming docks manually is no different on the iPhone 5's bigger screen, by the way.
Hope this helps and don't hesitate to ask any questions.