Takes all the pictures in the folder in, scales the image while maintaining aspect ratio as big as it can without exceeding 650 x 350, and then creates a 700 x 400 image of the same name in the out folder with a black background.

ImageScaleBG.go

package main

import (
	"image/color"
	"io/ioutil"
	"log"

	"github.com/disintegration/imaging"
)

func main() {
	pics, err := ioutil.ReadDir("in")
	if err != nil {
		log.Fatalf("Unable to read directory. Error: %v", err.Error())
	}

	for index := 0; index < len(pics); index++ {
		inPath := "in/" + pics[index].Name()
		outPath := "out/" + pics[index].Name()

		i, err := imaging.Open(inPath)
		if err != nil {
			log.Fatalf("Unable to open file. Error: %v", err.Error())
		}
		if i.Bounds().Dx() >= i.Bounds().Dy() {
			i = imaging.Resize(i, 650, 0, imaging.Lanczos)
		} else {
			i = imaging.Resize(i, 0, 350, imaging.Lanczos)
		}

		bg := imaging.New(700, 400, color.Black)

		saveImg := imaging.PasteCenter(bg, i)
		if err = imaging.Save(saveImg, outPath); err != nil {
			log.Fatalf("Unable to save file. Error: %v", err.Error())
		}
	}
}