Here is what the writexml does with your image:
case "System.Byte[]":
// special case, base64encode it
byte[] valueToConvert = (byte[])propertyValue;
valueAsString = Convert.ToBase64String( valueToConvert, 0, valueToConvert.Length );
break;
and what the read xml calls:
case "System.Byte[]":
toReturn = Convert.FromBase64String(xmlValue);
break;
But now you should keep in sync with your calls:
either you keep up with the entities (adapter save/fetch)
either you work with the xml (writexml/readxml)
But you should not use writexml to store it in the other image column. Writexml deals with the whole entity, of a given type. You cannot put anything else behind than a readxml from the same entitytype.
Then you can pass things between the entities, but they can't pick up the information into each others serialized bit.