Pixelation when loading a greyscale image

I've got a tiff image that is greyscale, and when I load it using Image.FromFile, the quality of that image is very pixelated. However, if I save the tiff as indexed in Gimp beforehand, then loading that image with Image.FromFile is no longer pixelated.

Is there anyway I can load a greyscale image at the original quality?

[edit] Note that when I convert the image to indexed, the palette is still only 256 entries. Viewing both the greyscale and indexed image in another app (eg. xnview) shows both versions of the image in the original quality. My conversion isn开发者_如何学运维't causing data loss. The pixelation is happening when loading it into my C# app.

Thanks for any help, Dan.