Which one of those two tables has the metadata of the multimedia-file to include? I presume 'multimedia' represents a photo or video, and 'multimediacollection' represents a collection of 'multimedia'? Then he did exactly (well, a constraint would be nice...) the right thing: this way you can reuse a photo in different collection. If a photo points to a collection instead of the other way around a photo (or video) can be used only once.
If you don't want to reuse collections should dispense of the 'multimediacollection'-table and add a 'entitimultimedia'-table: ID (PK), ENTITY_ID (FK to Entity.ID), MULTIMEDIA_ID (FK to Multimedia.ID). ID is not really needed here but can be included because of coding standards or a desire not to have a PK on multiple columns. If collections can be used more than once you add a MULTIMEDIACOLLECTION_ID (FK to MultimediaCollection.ID) field to the Entity-table.
Well, if this were a new application anyway. At the moment you have to make due with what your predecessor wrought :P
What I don't get is why MultimediaCollection has both an ID and a MultimediaCollection_ID...