If you have the image as bitmap, you can use this
private byte[] GetByteArray(Bitmap bitmapImage)
{
var byteArray = (byte[])TypeDescriptor.GetConverter(bitmapImage).ConvertTo(bitmapImage, typeof(byte[]));
bitmapImage.Dispose();
return byteArray;
}
And in fact, for each situation you don't have to be putting the same default image repeatedly in the database, so, I think you should just store it once and retrieve it for every case where there is no default image. With regards to storing null image in the respected column, I believe a better practice is to have a separate table with two columns where you put the entity primary key as foreign key and the second column will contain the image, this way, any item with no record in this table has no image of its own, sow we get the standard default image that is store in just one location.