I don't think the Herald is still going, or if it is, I've never seen it in any shops. The History Centre has the archives of the Herald on Microfilm, so if you eventually get a date of birth, it may be possible to find the issue that the photo was in.
I think its probably going to be the death certificate for Ada which will give more clues. If Walter was still alive, it may say that she was the wife of Walter, or if he was dead, it should say Widow of Walter.
Just another quick question. Does your father's marriage certificate say whether his father was deceased? (Usually it just has "dec" in brackets or something like that)
Glen