Edit in JSFiddle

var imageLoader = document.getElementById('imageLoader');
var originalImage = document.getElementById("original-image");
var filteredImageCanvas = document.getElementById("filtered-image");
var filterChanger = document.getElementById("filter-changer");
var imageUploaded = false;

// Handle image upload into img tag
imageLoader.addEventListener('change', function(e){
		var reader = new FileReader();
    
    reader.onload = function(event){
        originalImage.onload = function(){
             console.log("Image Succesfully Loaded");
             imageUploaded = true;
        };
        originalImage.src = event.target.result;
    };
    
    reader.readAsDataURL(e.target.files[0]);   
}, false);

filterChanger.addEventListener("change", function(e){
	var filter = filterChanger.value;
  	
  if(imageUploaded && filter != "none"){
  
  	// Apply filter
  	LenaJS.filterImage(filteredImageCanvas, LenaJS[filter], originalImage);
  }
}, false);

<p>
Upload an image from your desktop and select a filter from the select:
</p>
<input type="file" id="imageLoader"/>
<label for="filter-changer">Select a filter to apply</label>
<select id="filter-changer">
  <option value="none">None</option>
  <option value="red">Red</option>
  <option value="gaussian">Gaussian</option>
  <option value="grayscale">Grayscale</option>
  <option value="highpass">highpass</option>
  <option value="invert">invert</option>
  <option value="laplacian">laplacian</option>
  <option value="prewittHorizontal">Prewitt Horizontal</option>
  <option value="prewittVertical">Prewitt Vertical</option>
  <option value="roberts">roberts</option>
  <option value="saturation">saturation</option>
  <option value="sepia">sepia</option>
  <option value="sharpen">sharpen</option>
  <option value="sobelHorizontal">sobel Horizontal</option>
  <option value="sobelVertical">sobel Vertical</option>
  <option value="thresholding">thresholding</option>
</select>

<br>

<img id="original-image" />
<canvas id="filtered-image" />