Add attributes to project files

When saving a project file, ArtPaint could add attributes:
* Media:Thumbnail
* Media:Width
* Media:Height

There could be more.
The ArtPaint settings could have an option to set a "META:Artist" that'll be added to all projects. Are there more?
The save image/project dialog could have an option to set a "META:Comment". Maybe optionally override the META:Artist of the Settings. More?
