It seems that there is no easy way to draw something onto a background image and save it with original size and resolution in Flutter. Flutter doesn't support double buffer so we can't draw directly onto an image. It has toImage(pixelRatio: dpr) or PictureRecorder but the image acquired from both is limited to widget/screen size, not the size of the original image size.
Does anyone know a package that can draw directly onto an image in Flutter?