what software packages do u have?
u can AA to transparent but it doesn't seem stylistically suitable here.
Some things u can do in terms of outlining:
- fill ur canvas with 50% grey when developing sprites. display them like this too when u post.
- increase contrast inside the sprite so the outline isn't so harsh.
- avoid pure black outlines.
- I often lean towards a dark redish hue for outlines as this warmer colour tends to separate the sprite from coolor backgrounds. Typically somewhere around 50% saturation is strikes a good balance between a range of backgrounds.
- use the outline colour selectively in the most shadowy areas or where u really need a dark line to separate forms inside the sprite because u can't use colour/value separation.
where the light is hitting eg top of head, switch up to the darkest shade up from the outline. this is where the ramp ur using is important as mentioned above re contrast.
u can even go a shade lighter in the brightest areas.
the concept here is that bright parts of the sprite already pop against dark to mid-bright backgrounds, so u only need a mid-bright outline to separate it from bright backgrounds.
finding the balance helps make the lines less harsh while still serving their purpose.