Create Bitmap from byte[] gray levels

Hello,

I have a byte[] array containing values of 0-255 representing gray levels of
a picture. This has no common format (.bmp, .jpg, ...) it only stores the
bytes, one per a pixel.
I converted it from original matrix byte[,] so I know its real dimensions.

Now, I would like to display this as a bitmap. I tried to create
MemoryStream from the byte[] array, but then the call to Bitmap.FromStream()
exceptions with "Invalid parameter".

many thanks Ondra.