Removes validating check for default image.

- the ImageLoader checks that an initial image is set, which will be replaced when the target image has been acquired.
- that check has been removed; it may be the user doesn't wish to display anything.
- a simpler factory method has been added; minimum parameters should be context; used to get display dimensions (for ImageView); it may be a parameterless factory method is not appropriate.
- using this simpler method _without_ setting a default image results in odd behaviour to be investigated (but is no different from using the parameterless function, and not setting the default image too).