Starting with this:
- Do a fuzzy selection on the background of the image (it can be useful to increase the size of the layer first to have some margin all around)
- Select ➤ Grow by a couple of pixels, so that the selection bleeds inside the design
- Select ➤ Invert so that the selection is now slightly inside the design
- Add a new layer
- Bucket fill the selection: this will give you something that has the overall shape of the design, like this:
- Back to the original layer: Layer ➤ Transparency ➤ Alpha to selection, to obtain a selection shaped after the original
- Back to the added layer: Edit ➤ Clear to delete the original design.
Quote:I've tried the obvious solution on my own work, which is to create a new layer, fill with black and then use the Fuzzy Select or Select By Color tools to highlight the original black outline on my image, and then delete from the black layer to get the black "opposite" image, but it creates very rough and pixelated edge.
See the Background removal or replacement on text, logos and other computer graphics thread for an explanation.