ffmpeg -i image.jpg -vf "scale=1024:1024:force_original_aspect_ratio=decrease,pad=1024:1024:-1:-1:color=white" -q:v 1 tex_image.jpg

# longest side, so basically original size with padding to square
ffmpeg -i image.jpg -vf "scale=max(iw\,ih):max(iw\,ih):force_original_aspect_ratio=decrease,pad=max(iw\,ih):max(iw\,ih):-1:-1:color=white" -q:v 1 tex_image.jpg