If you are talking about the images folder that construct creates after exporting (which contains the images of the sprites etc.): you don't need to change anything in your event system. The swapped image will appear in the game (possibly it could lead to errors, if the swapped in image has the wrong format).
If you are talking about imported files: if you created a subfolder, you load the sprite from URL with: "subfolder/image.png", if not you can just load "image.png".
If you want your users to dynamically store pictures in the game, you could build an editor in your game. A textfield could be used for pasting in urls from images, a file chooser for local images. Load the images to a placeholder sprite and then use the canvas plugin (or paster plugin) to paste the images onto the canvas/paster and save the base64-string of the contents to webstorage (or a database of yours). Everytime your user starts the game, the value will be retreived and loaded into the sprite (if it exists).
Modifying animations will not be practical.