Hmmmmm.....
If I have images that were brought in through the Image Editor and double-click on these Texture Image nodes in Node Editor, the Texture from file field gets focus.
If I make a brand spankin' new Texture Image node and double-click it, the Texture from file field gets focus.
If I have a LW procedural plugged into Texture Image and double-click, the Texture from file field is grayed out, which is expected.