Nothing, your texture is probably packed or has zero users and no fake user. Packed images are stored inside your blend file and not on disk so they cannot be changed by an external program. Therefore it makes no sense to watch them. If you have enabled automatic addition in the add-on preferences (on by default) the texture will be enabled as soon as it is unpacked to disk.
If your texture is stored on disk it doesn’t matter which program is used to change the texture. As soon as you save/export your image or texture set to disk, TextureWatch will detect the change and update the texture inside Blender.