Here's the official Mathworks solution: http://www.mathworks.com/matlabcentral/answers/98593-how-do-i-display-an-image-on-a-gui-component-eg-pushbutton
For your #2. Just set a flag
global userOpenedBeam;
userOpenedBeam = true;
and check it wherever you need to
global userOpenedBeam;
if userOpenedBeam
uiwait(warndlg('You cannot open beam project again!'));
return;
end