saveppt2 giving error when saving multipe iamge file

Hi All, I am trying to save multiple images in a ppt , but on executing the command I am getting this Error:

Error using horzcat Dimensions of matrices being concatenated are not consistent.

Error in saveppt2>validateInput (line 785) error('validateInput:UnknownParameter',['Unknown Parameter: ' argsIn{i}]);

Error in saveppt2 (line 207) addlParms=validateInput(varargin,validParameters);

Code which I executing is as:

        for i = 1:numel(pngfiles)
         im{i} = imread(pngfiles{i});
        imrgb{i} = rgb2gray(im{i});
         imrgb_z{i} = imrgb{i}(100:350,280:580);
         saveppt2('Image.ppt',imrgb_z{i},'title',sprintf('Image. %d',i));
      end

end

Please let me know where I am doing wrong.

Thanks