Maybe it was how you saved it after. For example: if your source was jpg and you save it a jpg the quality will actually degrade. Saving a file as jpg multiple times will degrade the quality each time. It's safest to save it as png.
Anyway, I'll do a quick tutorial for making an avatar w/o quality loss, and should look better if scans are a bit dirty; Mangapanda scans for example.
I just downloaded this to try and help, so bear with me. Lol
Source:
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Anyway, from there we crop it down to the rough* area we want. Do part, either height or width, to where you want it, do this to the smallest side. Then resize it with that one as the 200 or 150. You then crop the rest to make it 150x200. You already know this, I'm sure...
If there are dialogue boxes not touching the character or anything just go over them with a white brush.
Rectangle should be selected. Brush width 2. Style should be solid line. Fill leave as solid color. Do it so it's a 2px white rectangle around the outside.
Change primary colour to black. Shapes thing again. Brush width 1. Change the solid line to dotted. Draw over the outside of the white one and you should get a dotted border.