You don't really need any specific programs since the Lua files are just text, most authors use Notepad++ or other basic text editor. The only image formats currently supported by the Lua controls are JPG and TGA files, so Paint.net or similar image editor will work fine. There are a couple of stickies on the lotro.com forums under the Lua section that pertain to getting started with Lua - the "introduction" post has links to the API and some Turbine samples: http://forums.lotro.com/showthread.p...Lua-UI-scripts
Note that the dowloads are in 7Zip format but there's also a link to the free 7Zip utility in that post.
I would recommend reading through the "Current API Issues & Discussion" forum (under the Lua section on forums.lotro.com) to get familiar with the known bugs/pitfalls and some of the existing workarounds. You might want to also check out the "Plugins & Requests" section to see what plugins have already been developed.