questo è il codice che uso io

Codice PHP:
private bool ResizeAndSave(Stream imgStr,int Widthint Height,string FileName)
{    
    
//creo il bitmap dallo stream
    
System.Drawing.Image bmpStream=System.Drawing.Image.FromStream(imgStr);
    
int OriginalWidth bmpStream.Width;
    
int OriginalHeight bmpStream.Height;
            
    if (
Height == 0)
    {
        
Height = (OriginalHeight Width) / OriginalWidth;
    }
    if (
Width == 0)
    {
        
Width = (OriginalWidth Height) / OriginalHeight;
    }
    
//creo un nuovo bitmap ridimensionandolo            
    
Bitmap img=new Bitmap(bmpStream,new Size(Width,Height));           
    
//salvo l'immagine ridimensionata
        
string nomeMini Server.MapPath("public\\\\"+FileName);
    
img.Save(nomeMini,System.Drawing.Imaging.ImageFormat.Jpeg);        
            
    return 
true;