Redraw a transparent PNG into one with an opaque background. This particular code will redraw with a white background.

redraw.go

package main

import (
  "image"
  gcolor "image/color"
  "image/draw"
  "github.com/go-playground/colors"
)

func redrawImage(imgBytes []byte) error {
    originalImg, _, err := image.Decode(bytes.NewReader(imgBytes))
    if err != nil {
      return err
    }

    if _, _, _, a := originalImg.At(0, 0).RGBA(); a == 0 {
      // default to white; A=0xff is opaque and not white
      bgColor := &colors.RGBAColor{R: 0xff, G: 0xff, B: 0xff, A: 0xff}

      // redraw images with transparent background
      // otherwise only a black image is generated when decoded above
      // applies to PNGs only. JPEGs don't support transparency
      dst := image.NewRGBA(originalImg.Bounds())
      draw.Draw(dst, dst.Bounds(), image.NewUniform(gcolor.RGBA{
        bgColor.R,
        bgColor.G,
        bgColor.B,
        0xff,
      }), image.Point{}, draw.Src)
      draw.Draw(dst, dst.Bounds(), originalImg, originalImg.Bounds().Min, draw.Over)

      originalImg = dst
    }
  
    // do something with the image
  
    return nil
}