If you want to upload to a thread you have to hit post reply. You can't do it from quick reply. Its located under the last posters post on the left side.
If your wanting to add a picture to a thread from your profile there are two links given at the bottom of your picture in your profile. You'll want to copy the img code link and paste it to whatever thread your wanting to add it to.
That is actually the recommended way because it saves time space and your picture won't get deleted after so long as long as you don't move it from your profile