You've probably gotten the dog image assigned to Lambert1 wich is the default shader.

Go to window>rendering editors>hypershade and check if
Lambert1 has the dog image on it. If so go to Edit>Delete all by type>textures and it should become gray. There may be a better way but that's what I can give you.

Now just create a new lambert material and assign you image to that and assign the new material to your plane.