Added description to image-set-items image case

Added clarification to the image-set-items, explicitly requiring the use of an image-item-core object when the image set item is an image.

assigned to @karsten.schimpf

requested review from @tschoening

requested review from @soschindler and removed review request for @tschoening

requested review from @tschoening and removed review request for @soschindler