I just tried it: I took an existing JPG Image, loading into PaintShopPro, saved it as a TIFF file.
Then I added a picture box to a form, and in the form constructor I loaded the file:
Image i = Image.FromFile(@"D:\Temp\mypic.tiff");
pictureBox1.Image = i;
It was not blurred, or otherwise degraded in quality.
So, what are you doing that is different to what I did?
FYI, the image I used was:
3648x2736, 180dpi, 24 bit colour, LZW compression, RU=2, PI=RGB, 15.2Mbyte