(Caveat: I haven't tried running any of this code)
What are you expecting to happen when you set the ButtonDownFcn? It doesn't look like you're actually setting a function there, but are evaluating the function instead. Why not modify it like this?
appel = @(h,e)show_in_figure(h1,h2);
% ^^^ this capture the default inputs of a callback: the object handle (see doc:gcbo) and the event data
set(im1, 'ButtonDownFcn', appel); % ButtonDownFcn modified to show figure on the larger axes
set(im2, 'ButtonDownFcn', appel); % ButtonDownFcn modified to show figure on the larger axes
function show_in_figure(h1, h2)
handles=guidata(h1);
copyobj(h1,handles.handle_to_axes_large1)%copy image object to second axes
copyobj(h2,handles.handle_to_axes_large2)%copy image object to second axes
%if you want to copy the axis labels etc you can put that code here
end
Do not rely on gca inside GUIs, as you can always determine the handle. You can use gcbo and gcbf if you want, but you should never need findobj.
For general advice and examples for how to create a GUI (and avoid using GUIDE), have look at this thread.
