I would first try to do the upload with Firefox or Chrome. If those work, the issue is IE9.
If you are using DotNetNuke 5.6.2, you can try the fix described in Joe Brinkman's blog.
You can also configure IE9 to run in compatibility mode:
1. Hit F12 to open the developers console.
2. Click on “Browser Mode: IE9″ at the top.
3. Select your browser.
You can also add a meta tag in your skin:
<meta http-equiv="X-UA-Compatible" content="IE=EmulateIE7" />