Since you did not mention it was GWT related, I moved it.
Like you rightly mentioned, GWT does not lend support to such operations.
You are left with essentially two options
1) Send the image to be processed onto the server
2) Use something like an
applet embedded inside your GWT page. Of course you would need to sign the applet and the user still would need to give the applet permission to read from the disk.
I personally would go with #1