At first, this would be l’maaseh a nafka minah if giving a picture is assur or ein roy l’asos (translation: Roy shouldn’t do it).
If it is assur, then ein issur chal al issur. If not, then it’s an issur of sheker.
However, after much hisboddus (translation: long work day), I think it is like this. The issur is to give your granddaughter’s picture, not another woman’s picture. And if you want to say it is being nikshal the people, that is only on the tzad they don’t recognize the person – because if they do then they have internet and know celebrities and lifnei es deightee there is no issur and it will be mavir if the shidduch is bad – a tzroech gadol. And even if they don’t recognize the photo, therefore a potential nikshal of seeing the picture, there is no problem of lifnei ever because maybe they are being somech that giving pictures is only not fitting, not mamish an issur.
Therefore, you have to what to rely on cosidering the hefsid meruabah. B’frat, your granddaaughter is in time of the shidduch crisis -so it’s a horaas shaa. M’stama. About the din sheker, some say that since photoshoping is so common then like parve creamer with chicken it loses the issue.
Another point added in another thread is if what is usually considered trolling is allowed from Tu B’shevat or only until Purim. Even if allowed, from when? Considering, DY posted this 22 hours ago b’erech 8:30-9pm EST that is Tu B’Shevat even on the west coast it’s after shkiyah. However, in Hawaii – it was still the 14th of Shevat. The question is it based on the poster or since a forum is accessible in the whole world – every place must be tu b’shevat. Accordingly, DY might still have the din of a troll. But the olam seems mekal from Rosh Chodesh Shevat – relying on the opinion of Bais Shammai.