To assign the image

It seems you might be looking for a way to copy the image data from server side to client side 'as is', but I don't think you can in that way.

I see 2 options:
1) use the retrieved byte array to show a (thumbnail) image using a data uri.
2) save the data to disk and assign the src of the image to that.

For the 1st option, you could use:

aspImagePreview.Attributes["src"] = "data:image/jpg;base64," + Convert.ToBase64String(bArray);

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If the pictures are big (bigger than the 290x190 you're showing) then it would be beneficial to generate a thumbnail image:

            aspImagePreview.Attributes["src"] = "data:image/jpg;base64," + Convert.ToBase64String(GetThumbBytes(img));
        }

        public byte[] GetThumbBytes(System.Drawing.Image img) {
            System.Drawing.Image thumbnail = img.GetThumbnailImage(290, 190, new System.Drawing.Image.GetThumbnailImageAbort(ThumbnailCallBack), IntPtr.Zero);
            MemoryStream thumbStream = new MemoryStream();
            thumbnail.Save(thumbStream, System.Drawing.Imaging.ImageFormat.Jpeg);
            byte[] bArrayThumb = new Byte[thumbStream.Length];
            thumbStream.Position = 0;
            thumbStream.Read(bArrayThumb, 0, (int)thumbStream.Length);
            return bArrayThumb;
        }

        public bool ThumbnailCallBack() {
            return true;
        }

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you can use a regular img instead of server side by the way, just have a function returning the 'src' string and in the html something like:

<img id="aspImagePreview" src="<%= GetThumb() %>" alt="Preview" height="190" width="290"/>

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Thanks a lot Robert. How can I adjust my own codes above to save the image into the disk?

html could then simply be something like:

<img id="aspImagePreview" alt="Preview" height="190" width="290" src="tmp/output.jpg" />

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In the previous post, I used 'tmp' directory because you will probably have to set write permissions for web user(s) to that directory. Also you may want to think of a solution to delete the files after some period or maybe when session ends but never tried that...

Thanks a lot.
I have this Image item on the page
       <asp:Image ID="aspImagePreview" runat="server" AlternateText="Preview" Height="190px" Width="290px" src="~/output.jpg" />
       ...

and I have these codes within Page_Load      
                conn.Open();
                SqlCommand cmd = new SqlCommand("SELECT photo_file FROM tab2 where LTRIM(RTRIM(user_abbr))=@par_id", conn);
                if (sv_userid != null)
                {
                    cmd.Parameters.Add("@par_id", SqlDbType.VarChar).Value = sv_userid.Trim();
                }
                else
                {
                    cmd.Parameters.Add("@par_id", SqlDbType.VarChar).Value = "";
                }
                byte[] bArray = (byte[])cmd.ExecuteScalar();
                using (FileStream fw = new FileStream(Path.Combine(Server.MapPath("~/"), "output.jpg"), FileMode.Create))
                {
                    fw.Write(bArray, 0, bArray.Length);
                }
                ...

but when running the deployed project, I don't see the Image is displayed there, while for the relevant record, there is really one Jpg file inside and I am able to display it by other ways (directly from the database).