var myIcon = L.icon({
      iconUrl: '<img src="feature.properties.description">',
      iconSize: [40, 40]
      });

var geojsonLayerP = new L.GeoJSON.AJAX("https://raw.githubusercontent.com/marinespatialviz/marinespatialviz/master/litter.geojson", {
         pointToLayer: function(feature, yx) {
           var marker = L.marker(yx, {icon: myIcon});
           marker.bindTooltip('<p>' + feature.properties.description + '</p>')
           return marker;
         }
       }).addTo(map);