You can use Autoupload module
AutoUpload is a user interface (UI) enhancement that initiates
automatic upload of files minimizing the number of clicks required by
a user.
Currently, users must select files, then press the "Upload" button. We
found users often don't realize a button press is necessary and
mistakenly think their image is uploaded when it's not.
This module removes the extra button press and hides the "Upload"
button via JavaScript for a quicker, automatic file upload. When
JavaScript is not enabled, it falls back to the "Upload" button.
or you can check this post.