Jean-Fred,
That is an interesting example. In Wikidata terms this is of course a composite object. I would then describe it in two items, the parchment and the seal, where the seal is part of the parchment. I can understand that on Commons you might feel it's overkill to have two images, but for someone doing a quick comparison of seals it may be nice to make a derivative image and rotate the image to straighten it out. Maybe there already is one, in which case the Wikidata "has part" property would be associated with the commons "gallery of one" inclusion under the "other versions" field.
Jane