Are you looking for your textures in the Texture dropdown menu in the UV Editor? If so, then that only shows the textures currently plugged into the material assigned to the selected object and even then it's only for preview purposes inside that editor. You'll have to plug your file texture into the desired channel on the material. Easy way if you already have the file node in your scene is to middle mouse drag and drop it from Hypershade into your shader's color channel for example.