Попробуйте это:
//example 1: converting from bitmap
Bitmap myImage1 = new Bitmap(@"C:\myimage1.bmp");
myImage1.Save(@"C:\myimage1.jpg", System.Drawing.Imaging.ImageFormat.Jpeg);
myImage1.Save(@"C:\myimage1.png", System.Drawing.Imaging.ImageFormat.Png);
//example 2: converting from pixels
Bitmap myImage2 = new Bitmap(10, 10);
//for loop to set some pixels
for (int x=0;x<10;x++)
for (int y=0;y<10;y++)
myImage2.SetPixel(x,y,Color.Blue);
myImage2.Save(@"C:\myimage2.jpg", System.Drawing.Imaging.ImageFormat.Jpeg);
myImage2.Save(@"C:\myimage2.png", System.Drawing.Imaging.ImageFormat.Png);