TImage32.SetBackgroundColor

procedure SetBackgroundColor(bgColor: TColor32);

Assumes the current image is semi-transparent.

uses Img32, Img32.Fmt.PNG;
...
img := TImage32.Create;
img.LoadFromFile('beetle.png');
img.SetBackgroundColor(clAqua32);
img.SaveToFile('beetle_aq.png');
img.Free;

Before:

After: