.slider1{ background-image: url(../../../img/secciones/slider_img_tum_1.jpg);} 
.slider3{ background-image: url(../../../img/secciones/slider_img_tum_3.jpg);} 
.slider15{ background-image: url(../../../img/secciones/slider_img_tum_15.jpg);} 
@media only screen and (max-width:550px){ 
.slider1{ background-image: url(../../../img/secciones/slider_img_tum_cel_1.jpg);} 
.slider3{ background-image: url(../../../img/secciones/slider_img_tum_cel_3.jpg);} 
.slider15{ background-image: url(../../../img/secciones/slider_img_tum_cel_15.jpg);} 
}