Make sure that your 'Image' variable is cast as a double and actually has a zero level-set.
Here is an example:
% Sample implicit surface
x=linspace(-5,5,201);
[x,y,z]=meshgrid(x);
F=x.^4 - 5*x.^2 + y.^4 - 5*y.^2 + z.^4 - 5*z.^2 + 11.8;
% Get zero level-set of F
[tri,vrt,col]=isosurface(x,y,z,F,0,z);
% Evaluate surface normals at vrt
N=isonormals(x,y,z,F,vrt);
% Visualize
figure('color','w')
h=patch('Faces',tri,'Vertices',vrt,'FaceVertexCData',col,'FaceColor','interp','EdgeColor','none');
set(h,'VertexNormals',N)
axis equal
view([-44 32])
camlight('headlight')
lighting phong
grid off
box on
colormap('jet')
