Original Cat

This what happens if you use a green background, but don't limit selection to the background color, colors wash out.

Better Cat Animation

What wrong here, is that you painted to background color, then scaled the image smaller, some pixels of mixed background/forecolor are left in the output.

Gimp - How to cut,scale and make transparent background for image

  1. Load image
  2. Select->Select by color, click on a background point.
  3. Colors->Color to alpha, select background color.
  4. Image->Scale Image, Scale Image to correct size.
  5. At this point, the alpha may be tinted, with Select by color still active, choose a pure color, and paint the background
  6. Colors->Color to alpha, select new background color.
  7. Save or Export the Image

Finally you get

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