For now, the picture views to generate are hardcoded in the AbstractPictureAdapter#addViews method.
The only way to have different picture views than the default ones is to put the Picture in a PictureBook.
It should be possible to define the Picture views as contributions on the ImagingService.
We need to handle the case where there are missing required picture views such as Medium, Small, Thumbnail, Original, OriginalJpeg, the ones Nuxeo relies on. If they are missing, add them using default values.
Picture views can be disabled and merged.