Image resizing - sometimes very poor quality?

I cannot peek into the .NET source at the moment, but most likely the problem is in the Image.GetThumbnailImage method. Even MSDN says that "it works well when the requested thumbnail image has a size of about 120 x 120 pixels, but it you request a large thumbnail image (for example, 300 x 300) from an Image that has an embedded thumbnail, there could be a noticeable loss of quality in the thumbnail image". For true resizing (i.e. not thumbnailing), you should use the Graphics.DrawImage method. You may also need to play with the Graphics.InterpolationMode to get a better quality if needed.

Change the last two lines of your method to this:

var newImg = new Bitmap(newWidth, newHeight);
Graphics g = Graphics.FromImage(newImg);
g.DrawImage(origImg, new Rectangle(0,0,newWidth,newHeight));
newImg.Save(this.GetBitmapPath(filename), System.Drawing.Imaging.ImageFormat.Bmp);
g.Dispose();

If you're not creating a thumbnail, using a method called GetThumbnailImage probably isn't a good idea...

For other options, have a look at this CodeProject article. In particular, it creates a new image, creates a Graphics for it and sets the interpolation mode to HighQualityBicubic and draws the original image onto the graphics. Worth a try, at least.