Go to
http://www.lotrointerface.com/downloads/cat5.html
And download the latest UI Skin Art pack (there's also other interesting stuff there, but many are getting outdated as we speak).
In that file you can find all the things we can edit. The file includes all the original .tga files and Skindefinition.xml which have the corresponding ArtAsset tags plus SkinDictionary.txt that includes most of the panel code.
Best way to find out what image goes with what tag is to browse the images in an image browser (like
FastStone IV) and compare the titles.
For coding
Notepad++ is a pretty good proggy because if supports color coding and thus making things a bit more easier to edit.