W I S C H N E R

The issue is to size the picture to fit into given window without distorting it. I found many über complicated solutions on the internet so I ended up writing my own. Here's the algorithm in prose:

Find the factor by which you need to multiply your picture's width and height. Try using outer height / inner height and if the width doesn't fit, use outer width / inner width.

Here's the code fragment.

private float ScaleFactor(Rectangle outer, Rectangle inner)
{
    float factor = (float)outer.Height / (float)inner.Height;
    if ((float)inner.Width * factor > outer.Width) // Switch!
        factor = (float)outer.Width / (float)inner.Width;
    return factor;
}

To fit picture rectangle (pctRect) to window rectangle (wndRect) call it like this

float factor=ScaleFactor(wndRect, pctRect); // Outer, inner
RectangleF resultRect=new RectangleF(0,0,pctRect.Width*factor,pctRect.Height*Factor)