So you have a nice new fancy icon to add as an inventory icon, maybe even for a weapon part.
These should usually have black outlines are per the base game models.
In GIMP the best option is to use the Drop Shadow option.
Example: 512x512 icon, we isolate the icon and on a transparent background. If possible make the icon slightly smaller than the space, but tbh it won't matter a great deal if the shadow touches the edges.
Choose the Drop Shadow filter under Light and Shadow.
The defaults do, well, a shadow. But we want it to be a bit different.
We use these settings:
- X/Y: 0 - this centres the shadow not offsets it
- Blur Radius - 2.0 - less "fade" and transparency at the edges of the shadow. 2.0 is a small amount.
- Grow radius: 8 - This gives the black thickness. Increase or decrease to suit. 8 seems pretty reasonable.
- Opacity: 1.5 - This makes the 8 width line mostly opaque
- Clipping: Adjust: Default (Adjust)
- Blending mode: Defaults (Replace/100.0)
This is the final effect:
Save this by hitting OK it will add it to the image.
It can sometimes look a bit odd on things with transparency in built but it isn't the worst, and could be edited to suit (eg cut the icon off where transparent parts start).