canvas.toDataURL(type, encoderOptions);　

<input id="sendImage" title="send a picture" type="file" accept="image/*" onchange="imgChange(this)">
 <input type="hidden" value="" id="send_image_value">
 <img id="showLoadingimg" src="'+CHAT_SITE_URL+'/templates/default/images/loading.gif" style="position:relative;left:200px;top:200px;z-index:999;display:none;">

function imgChange(e){
  //檢查是否有文件被選中
    if (e.files.length != 0) {
      var file = e.files[0],
      fileType = file.type,
      reader = new FileReader();
      if (!reader) {
        e.value = '';
        return;
      };
      var size = file.size;
      var max_size = 2*1024*1024;
      if(size>max_size){
        e.value = '';
        $("#send_alert").html('file is too large(>2M)');
        return;
      }
      $("#showLoadingimg").show();
      reader.onload = function(e) {
        //讀取成功，顯示到頁(yè)面并發(fā)送到服務(wù)器
        e.value = '';
        var org_img = e.target.result;
        var image_base64 = org_img;
        if(size>1024*80){//>80K的
          var img = new Image(); 
          img.src = org_img; 
          img.onload=function(){
            var scale = 1;  
            if(this.width > 300 || this.height > 300){   
              if(this.width > this.height){  
                scale = 300 / this.width; 
              }else{  
                scale = 300 / this.height;  
              }  
            } 
            var canvas = document.createElement("canvas"), drawer = canvas.getContext("2d");
            if(scale!=1) {//按最大高度等比縮放
              img.width *= scale;
              img.height *= scale;
            }
            canvas.width = img.width;
            canvas.height = img.width * (img.height / img.width);
            drawer.drawImage(img, 0, 0, canvas.width, canvas.height);
            var tmp_code = image_base64 = canvas.toDataURL(fileType);
            if(tmp_code!='data:,'){
              image_base64 = tmp_code;
            }
            img_send(image_base64);
          }; 
             
        }else{
          img_send(image_base64);
        }
         
      };
      reader.readAsDataURL(file);
    }
  }
  /**
   *為將圖片賦值給消息
  **/
  function img_send(image_base64){
    if(image_base64!='data:,'){
      $("#send_image_value").val(image_base64);
      send_msg();
      $("#showLoadingimg").hide();
      $('#sendImage').val("");
    }
  }　　

$("#send_image_value").val(image_base64);
send_msg();