image: API

<pre>The package comment should give a bit more introductory material but less than the blog
post has.

UnknownFormatErr should be ErrUnknownFormat or ErrFormat.

There are mentions of RGBAColor (etc) in doc comments that should say color.RGBA.

The doc comment on image.Repeated mystifies me.  It sounds like image.Translated to me. 
Where is the repetition?</pre>
