Thank you! Really glad you liked it :)
The image downloading uses the JavaScriptBridge singleton, below is my function, only works on web exports!
func _download_image_as_png(img: Image, filename: String) -> void:
# Encode PNG -> base64
var png_bytes: PackedByteArray = img.save_png_to_buffer()
JavaScriptBridge.download_buffer(png_bytes,filename+".png","image/png")