getimage expects either no arguements or a single graphics handle . It does a findobj 'type' 'image' and extracts the cdata of the first one it finds. You should not be passing two arguments .
You did not image() or imagesc() or imshow() in those axes handles so getimage would return empty .
guidata(object, data) chases the given object through the graphics hierarchy to find the parent figure and replaces all of the figure's gui data with the given data. The gui data for a figure is stored as one of the properties of the figure by way of setappdata so guidata is aa shortcut for setappdata . Convention has guidata used with a struct but that is not strictly necessary .
You call guidata twice with different axes handles . However they belong to the same figure so they refer to the same storage area so the second one is overwriting the first . However you never use the guidata so it does not matter .
I suspect that instead of using guidata there you want to display the images in the axes so that you can get them later with getimage .
guidata inside your callback is wrong because guidata cannot take 3 arguements .
You could
S.Img = Img;
guidata(varargin{1}, S)
and later use guidata to retrieve the updated S and get the Img field from it.
One thing to be cautious about is that when you create your callback during your assignment to S.pb then the S that will be recorded in {@pb_call,S} will be the S that exists at the start of the assignment before the assignment that creates S.pb is done . Therefore the recorded S will not include the pb field . It is safer to go through creating all of the objects in the struct without any callback settings first and only after all the struct fields have been filled in, go back through and set the callback properties .
As for why double(S.bg) fails, I have explained repeatedly that double applied to a graphics handle gives the old style numeric graphics handle , sort of like a street address . You are storing the results of the callback using guidata which is not going to associate it with any part of the buttongroup.
