When dragging an element to HTML5 canvas, the 'onDrop' can be triggered but you cannot add elements into canvas. Please rewrite the 'onDrop' code to draw some text on canvas:
$('.drop').droppable({
onDragEnter:function(){
$(this).addClass('over');
},
onDragLeave:function(){
$(this).removeClass('over');
},
onDrop:function(e,source){
var s = $(source).find('img').attr('title'); // get the source item text
var ctx = document.getElementById("wall").getContext("2d");
ctx.fillText(s, 10, 10);
}
});