Update: I noticed that the textures that Rhino did not accept were .jpg's. The first thing I tried was converting these to .png's, which is the format of the textures Rhino was accepting. This did not work. The next thing I noticed was that the unacceptable textures were of non-square dimension. I changed the dimension of the RyanCasualShirt.jpg texture to be square using photoshop and it worked!