Removing white space between cells when using images

[Sam Macdonald]

This is a pretty niche question, but I have a sheet where I've inserted a tile into each cell to make a map (of Hoenn from Pokemon Emerald), but even when I resize the row and column widths exactly there is still white space between the tiles, and I'd like to remove them if possible. Each of the tiles comes from a 16x16 png file, and in the screenshot each row is 21 pixels and each column 20 (Making it 21 increases the white space, I'm not sure why). Removing grid lines doesn't work either, and I'm not sure what else to try. 

Any ideas?