If you have an image column in your SQL table you can load the bytes using the following code.
int bufferSize = Convert.ToInt32(reader.GetBytes(7,0,null,0,int.MaxValue));
_ThumbnailImage = new Byte[bufferSize];
reader.GetBytes(7, 0, _ThumbnailImage, 0, bufferSize);
The first parameter is the column number, in this case 7, call GetBytes with a null buffer first to get the number of bytes needed.
Remember that an image column, holds bytes and therefore can contain any sort of document, not necessarily an image, could be a PDF