Help - How to get rid of those small pixels ?

[Boron]

How does this work with Gimp Thilock ?

I so far didn't find this option or tool

[Jurri]

Use the color-picker tool with zero tolerance to select every pixel of pure black in the picture; invert the selection; copy the selection; make new image, set the size right and choose transparent background; paste.

Other options include replacing pure black with e.g. white, then doing the necessary editing (whitening out the unwanted pixels), then replacing white again with pure black. At least I find white an easier background to work with than the 'transparent.'