In case you're really stuck on where to start,
you should break your task into two separate parts:
Uploading files to your applicationServing images to the client
To manage the file upload, have a look at the FileUpload helper module available in the Apache Commons library. Have a look here:
http://commons.apache.org/fileupload/using.html
Second, to serve images to the client, you need to create a Servlet, commonly known as an Image Servlet. If you search on this, you'll find lots of examples.