It may well be that the programme you are using is saving it with a lower compression. 492kb is far to large for the web.
What programme are you using?. Even the free Irfanview gives you an option of the level of compression. You should also consider resizing your picture.
Jpg`s should not be associated with Quick Time, your graphic programme should be the default for this type of file. If you don`t know how to change this, post again.