You're using the direct upload feature. You're charged bells for that.
If you want to avoid this, use Imgur or Photobucket to upload images and link the images via the IMG codes.
Today, 07:29 PM Thread Earned 6.3 Bells
Today, 07:21 PM Upload Spent 2.0 Bells
Today, 07:20 PM Upload Spent 2.0 Bells
Today, 12:31 PM Upload Spent 2.0 Bells
So you spent six bells for the uploads and got six back for the thread.