Use Small rendition for enrichments on Pictures

Today, on image enrichment we are always using the `file:content` blob.

Even if we do listen to `pictureViewsGenerationDone` event to start the enrichment, we do not use any of the Picture's renditions.

We need to split the image pipe to have a generic file:content and to Picture facet.
On Picture facet, we should use the `small` rendition blob by default.