.gal { width: 95%; margin: auto; overflow: hidden; }
.gal ul { display: flex; padding: 0; width: 100%; animation: cambio 20s infinite alternate linear; }
.gal li { width: 100%; list-style: none; }
.gal li img {  width: 100%; }

@keyframes cambio {
   0% { margin-left: 0; }
   20% { margin-left: 0; }

   25% { margin-left: -100; }
   45% { margin-left: -100; }

   50% { margin-left: -200; }
   70% { margin-left: -200; }

   75% { margin-left: -200; }
   100% { margin-left: -200; }

}