How are these intended to be used? I have a tileset where each tile image uses 100x100 pixels, and the tile is intended to be placed centered in a 50x50 cell, such that there is a 25 pixel border that overlaps with the surrounding cells. I tried setting the width and height to 50 and the margin to 25, and it looks like the images are being correctly identified in the tileset, but when I place them on the map they draw from the lower left, with 50 pixels of overhang all going to the right and up.
The margin and spacing determine only the regions in the tileset image that are to be used as tiles. In your case, your tile width and height should be 100x100 and with a 0 margin and spacing if you did not add additional space between your tiles that is not considered part of the graphics.
To achieve the centering, you would instead set a -25, 25 tile drawing offset on the tileset (you can set this in the Tileset Properties after adding the tileset).
