Resizing image but keeping aspect ratio
Your algorithm to calculate the final image dimensions could be improved by simplifying the logical process that you use to determine the final size.

Fundamentally, there are two dimensions you are interested in, the height, and width of the final image. There are two possible outcomes for the scaling, one where the scaling produces the target height, and the other where it produces the target width. The scaling that is used is the one which produces the smallest image. The size of the final image is determined by the scaling factor, and there are two factors:

float scaleHeight = (float)BOXHEIGHT / (float)original.Height;
float scaleWidth = (float)BOXWIDTH / (float)original.Width;


Now, we have two scaling ratios, which one produces the smaller image? The one that has the smallest scaling factor.

float scale = Math.Min(scaleHeight, scaleWidth);


Now you can simply do the adjustment:

resizedImage = new Bitmap(original,
         (int)(original.Width * scale), (int)(original.Height * scale));