Image Processing Help Request. IrfanView Experts?

I have a jpeg image with a dark blue foreground figure against a white background. I need to convert that to a black-and-white image for use as a mask.

Zooming into the raw image, the blue pixels deep into the foreground are of uniform color as are the white pixels deep into the background but near the boundaries between the two, random pixels have different shades of blue. Is it true that this is an artifact of the dithering in the raw jpeg image?

Using IrfanView to ImageMenu>"Decrease Color Depth" to 2 Colors [[black/white) [[1BPP) [[no dithering or grayscale), then saving that, then loading that into MSPaint, then applying MSPaint's Image>Attributes>Colors>Black and white before saving again, the resulting image has hairy edges.

Is there any way to avoid manually trimming those hairy edges? I suspect not. Is it true that once dithered, an image can only be undithered manually?

Sincerely.

Nevermind. Saving it from IrfanView as a BMP got rid of the hairy edges. Sorry.