You can do it like this:
Bitmap myImage = new Bitmap(50, 50); //assuming you want you image to be 50,50
Bitmap originalImage = new Bitmap("myPngSource.png"); //original image to copy
using (Graphics g = Graphics.FromImage(myImage))
{
g.DrawImage(originalImage, new Rectangle(0, 0, originalImage.Width, originalImage.Height));
}
MemoryStream ms = new MemoryStream();
myImage.Save(ms, ImageFormat.Png);
BitmapImage bi = new BitmapImage();
bi.BeginInit();
bi.StreamSource = ms;
bi.EndInit();
MyImageControl.Source = bi;
Or something like that, this is untested, and I just ripped it out of a little utility app I made a while ago. I hope it helps... You just need to change the width of the final image and do a loop over the g.DrawImage call incrementing the second parameter by the width of the originalImage. (i.e. if you want 5 repeats, do a for loop 5 times)
HTH
--Mark