[Nivlix]

You would need to create a context, and then create a set of texture frames for that context. Once you have the frames you could set the texture to whatever image file you wanted, set the width, height, position, and basically put it where you wanted it. Then you would set the layer to 1. I believe that most of the default UI elements would be drawn above a layer 1 frame. If you wanted for instance the L shape like LUI's bottom corners you would create two frames like this

mcf = UI.CreateContext("My Custom Frames")
lvert = UI.CreateFrame("Texture", "LeftVertFrame", mcf)
lhori = UI.CreateFrame("Texture", "LeftHoriFrame", mcf)

lvert.SetTexture = ("<SOURCE>", "<FILE>")
lvert.SetHeight(200)
lvert.SetWidth(50)
lvert.SetPoint("BOTTOMLEFT", UIParent, "BOTTOMLEFT", 10, 10)
lvert.SetLayer(1)

lhori.SetTexture = ("<SOURCE>", "<FILE>")
lhori.SetHeight(50)
lhori.SetWidth(200)
lhori.SetPoint("BOTTOMLEFT", lvert, "BOTTOMRIGHT", 0, 0)
lvert.SetLayer(1)

The above example is very basic and is not complete, but should give you a good starting point
<SOURCE> = the source of the file "Rift" if its a texture provided by trion or "Your Addon Name" for one you are providing
<FILE> = the location of the file relative to the source, so lets say you used a file, texture.png, that you provided and its in a folder called Textures which is inside your addon folder then it would be "Textures\\texture.png"