object LargeImage {get; set;}

private void ChangeCaptionImage()
{
  IPlugInWrapper wrapper = FrameworkApplication.GetPlugInWrapper("MyAddin_MyCustomButton");
  if (wrapper != null)
  {
    wrapper.Caption = "new caption";

    // ensure that T-Rex16 and T-Rex32 are included in your add-in under the images folder and have 
    // BuildAction = Resource and Copy to OutputDirectory = Do not copy
    wrapper.SmallImage = BuildImage("T-Rex16.png");
    wrapper.LargeImage = BuildImage("T-Rex32.png");
  }
}

private ImageSource BuildImage(string imageName)
{
  return new BitmapImage(PackUriForResource(imageName));
}

private Uri PackUriForResource(string resourceName)
{
  string asm = System.IO.Path.GetFileNameWithoutExtension(
      System.Reflection.Assembly.GetExecutingAssembly().Location);
  return new Uri(string.Format("pack://application:,,,/{0};component/Images/{1}", asm, resourceName), UriKind.Absolute);
}