A caution about small images
I posted something similar on the beta boards, but it bears repeating:
One caution about 1x1 textures...
If you use them for UI elements that are especially large or profuse (i.e. the LFF listbox background ) your UI performance will degrade a bunch. Why? The engine has to tile the image onto the surface, so if the surface is 200x20, that's 4000 copied pixels... if that's in a listbox with 100 items... yeesh.
Better to create your invisible textures at higher dimensions (e.g. 256x256) and let the engine clip the texture to the surface. Larger images (like maybe the character panel background) will still tile, but instead of 512x512 images at 1x1, it's be 2x2 images at 256x256.
And, I really don't know if there are mapable elements for the LFF listbox or character panel background, I was using them to illustrate the point.
You may even want to create various images of different sizes: Invisible_16x16.tga, Invisible_32x32.tga, etc. and use the one that best fits the image you are trying to replace.