The solution I mentioned was a quick solution for this drawing : flatten the cloud with all things in it and then blur (filters / blur / gaussian blur - value 1.5) and sharpen it with the curves (colours / curves - value see screendump on post above)
The blur arround the mouth you paste arises because everything you resized is resized, include the small blur on the edge of the original drawing of the mouth. In this case it is easy to correct the blur because the mouth has only one color : black.
Solution for tranparent layer :
- select the layer with the black mouth on it
- make a selection with the fuzzy select tool on the black color of the mouth
- shrink the selection (select / shrink - value 2 or more, depend how wide the blur is)
- make a path from the selection (select / to path)
- deactivate the selection (select / none)
- make a new transparent layer
- stroke path / stroke line (antialiasing) with the color of the mouth on the new layer (try different values to satisfaction)
- if ok, merge the layer down with the layer of the mouth
- if the mouth is a little grey in comparison with the other lines in your drawing, you can make it darker by making a duplicate from the mouth layer and then merge the two layers