If the texture is a power of 2 in size then showing a green box usually means that WoW can't find the texture in the location you provided or, you have placed the texture in the right place but you need to exit wow (all the way out) and re-start as WoW caches file locations only when it loads. Also it needs to be a .blp or a .tga with an alpha layer.
Code:
local frame = CreateFrame("Frame",ShowUF,UIParent)
frame.ufTexture = frame:CreateTexture()
frame.ufTexture:SetTexture("Interface\\AddOns\\WoWUI\\Background\\NaTUI")
frame.ufTexture:SetPoint("CENTER", UIParent)
frame.ufTexture:SetSize(256,64)