Image resizing class
PascalCase the method names and method params if you are feeling overly ambitious.

    // Set new width if in bounds
    if (onlyResizeIfWider)
    {
        if (ImageToResize.Width &lt;= newWidth)
        {
            newWidth = ImageToResize.Width;
        }
    }


FindBugs barks in Java for the above behavior... refactor into a single if since you are not doing anything within the first if anyways...

    // Set new width if in bounds
    if (onlyResizeIfWider &amp;&amp; ImageToResize.Width &lt;= newWidth)
    {
        newWidth = ImageToResize.Width;
    }


Comments here could be a bit more descriptive; while you state what the end result is I am still lost as to why that would resolve the issue.

    // Prevent using images internal thumbnail
    ImageToResize.RotateFlip(System.Drawing.RotateFlipType.Rotate180FlipNone);
    ImageToResize.RotateFlip(System.Drawing.RotateFlipType.Rotate180FlipNone); 


Maybe something similar to what is stated on this blog...

    // Prevent using images internal thumbnail since we scale above 200px; flipping
    // the image twice we get a new image identical to the original one but without the    
    // embedded thumbnail
    ImageToResize.RotateFlip(System.Drawing.RotateFlipType.Rotate180FlipNone);
    ImageToResize.RotateFlip(System.Drawing.RotateFlipType.Rotate180FlipNone);