MATLAB: Howto updated/manipulated image between the callbacks

gui

Hi,
I have several push buttons that performs image analysis (e.g., load an image, convert to gray scale). I wish to share the updated/manipulated image between the callbacks but I get an error.
Therefore, how can I update the image and share it between the callbacks?
Code for example:
function loadImageButton_Callback(hObject, eventdata, handles)
% hObject    handle to loadImageButton (see GCBO)
% eventdata  reserved - to be defined in a future version of MATLAB

% handles    structure with handles and user data (see GUIDATA)

I = imread('D:\Image\tele.jpg');
axes(handles.imageAxes);
imshow(I); 
guidata(hObject, handles);
function convertGSbutton_Callback(hObject, eventdata, handles)
% hObject    handle to convertGSbutton (see GCBO)
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    structure with handles and user data (see GUIDATA)
I = rgb2gray(I);
axes(handles.imageAxes);
imshow(I);
guidata(hObject, handles);
Thanks
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