Why do you need a 3rd party library to do this? I found this code with a google search, and it would be a simple matter to extend it to be more versatile with regards to which direction/position a 2nd image is to be stitched onto the original image.
private Bitmap MergeImages(IEnumerable<Bitmap> images)
{
var enumerable = images as IList<Bitmap> ?? images.ToList();
var width = 0;
var height = 0;
foreach (var image in enumerable)
{
width += image.Width;
height = image.Height > height ? image.Height : height;
}
var bitmap = new Bitmap(width, height);
using (var g = Graphics.FromImage(bitmap))
{
var localWidth = 0;
foreach (var image in enumerable)
{
g.DrawImage(image, localWidth, 0);
localWidth += image.Width;
}
}
return bitmap;
}