Antialiasing filter having no effect

[nekivud]

I have an image and am trying to apply the antialiasing filter.  I have followed the instructions here, and have gotten the filter to work correctly using the same sample image as in the instructions.  However, when I try to use my own image the filter has no effect, i.e. nothing visibly changes.  The image originally had a transparent background, which I thought might be the issue, so I converted the image to be only black pixels on a white background (exactly like the sample image), but the filter still has no effect.

I'm brand new to GIMP, and I'd appreciate any help with this, thanks!

Sample image from GIMP instructions:
   

My image: