sounds good... did you check for the IllegalArgumentException for this method ? http://javadesktop.org/lg3d/javadoc/0-7-1-latest/api/org/jdesktop/lg3d/sg/Texture.html#setImage(int,%20org.jdesktop.lg3d.sg.ImageComponent
It seems not to accept different sizes when the texture is already part of the scene... I'm not Java3D experienced but check for that scene status issue !
Plus it is not true that all sub-classes of Texture will accept the setImage() functionnality, try setDetailedImage() which will better reflect the Texture2D detailed features !