PNG images can have transparency information embedded in them and yours does. However, you discard it with your imshow call.
axes(handles.axes13)
[cdata, ~, transparency] = imread('logo4.png'); %get transparency
himg = imshow(cdata);
himg.AlphaData = alpha; %apply transparency to the image.
However, note that the background colour will be that of the figure, not the axes since imshow makes the axes not visible.
