The tiles are rendered with their origin in the bottom-left. You could give your tile a slight background color to make it easier to see where it aligns.
Since you want the wall centered on the tile, you can position it correctly either by adding empty pixels on the left side, or by using the “Drawing Offset” property on the tileset of the tile (this offset is applied to all tiles in the tileset, so it will require the wall tile to be in a different set than the ground tiles).
To access the “Drawing Offset” property, you go to “Edit Tileset” via the small button below the tileset view, and then choose Tileset -> Tileset Properties from the menu.
In the future I hope to support either custom alignment options and/or per-tile drawing offsets, but currently this isn’t possible.