I believe it's used as a variable to the image.
Yes, "original" contains pixels data, but depending on the original jpg file, those data can be "single", "uint16" ... and Octave only supports "uint8" and "double" data for textures. Hence my question: what does "class (original)" return?
If, as I'd bet, this returns anything but "uint8" or "double", then you can probably cast your data using "im2double" e.g.
figure ();
im = im2double (imread ("my_image_file.jpg"));
[X,Y,Z] = ellipsoid (0,0,0,10,10,10);
hs = surf (X, Y, Z, "FaceColor", "texturemap", "CData", im);
Pantxo