Don't click OK. You don't have to. Not yet.
- Hold down the Alt key and press the Print Screen key instead.
- Then open Paint or any other graphics program and
- paste the image into it and
- save it as a jpg file.
- Reply to this message and
- press the Go Advanced button below the message box
- Press the Manage Attachments button (a small window will open)
- upload the jpg picture.
- close that small window and
- press Submit Reply.