I agree with RichUK. I know nothing about that picture frame or the service behind it, but at the very least, it will require an always-on broadband connection (~$40/month) and this Momento service ($??) - not to mention electricity costs to keep these things powered. If something goes wrong with either the service or the device itself, she can't view any new photos. And what if she wants to see one of the old pictures again? She'll have to figure out that contraption or contact someone. Not to mention, outside of the Phillips picture frame, I have not heard good reviews about digital frames (though this may prove me wrong). It's a good idea... in theory.
I'm a geek and I love digital photography, but I still prefer actual prints. I'm sure your grandma would too.