(the bits per pixel method you are missing just details how the color information is stored per pixel)
As anthony suggested, a WriteableBitmap would be the easiest way – check out http://kodierer.blogspot.com/2009/11/convert-encode-and-decode-silverlight.html for a method to get an argb byte array out :
public static byte[] ToByteArray(this WriteableBitmap bmp)
{
// Init buffer
int w = bmp.PixelWidth;
int h = bmp.PixelHeight;
int[] p = bmp.Pixels;
int len = p.Length;
byte[] result = new byte[4 * w * h];
// Copy pixels to buffer
for (int i = 0, j = 0; i < len; i++, j += 4)
{
int color = p[i];
result[j + 0] = (byte)(color >> 24); // A
result[j + 1] = (byte)(color >> 16); // R
result[j + 2] = (byte)(color >> 8); // G
result[j + 3] = (byte)(color); // B
}
return result;
}