Tile objects only refer to a single tile. I guess you’ve got images in your tileset that actually occupy more than a single tile in the set. Currently placing such images as tile objects is not supported.
There is one work-around you can use to be able to place larger images as a single tile object: store the tile as its own image and add it to an Image Collection tileset (you can choose this option when creating a new tileset). This type of tileset refers to each tile image individually and can contain tiles of different sizes.
In the future I do hope to support also images of various sizes as part of a single tileset image, which is covered by the following issue on GitHub: