First of all, you should know that images (and another kind of items) are hierarchically organized into categorized and sub-categories according to available image tags. It's like folders on your computer.
You can filter results by adding tags from any of this areas:
"Pinned Categoies" list:
Available "Nested Categories" / "Tags" panel:
Selected image tag list:
"Type & Hit" search bar:
Added tags will be shown on the "Desired categories / Selected Tags" panel:
You can remove tags from the "Desired categories / Selected Tags" list by clicking on the desired tag. Or clear your search criteria by clicking "Clear Path" link.