No it's not safe to say. Other means would be needed to confirm when the picture was taken; it would be nice if metadata were attached to the Twitter picture file.
I'm not asserting that the picture was taken in December 2012, but am saying that leaf colour and coverage or lack of snow should not be taken as proof that it wasn't.
It was possible for leaves and grass to be green in December 2012. Historical daily weather data is available on line. The Pearson/Mississauga station shows that 2012 was above average temperature including December 2012. December precipitation was mostly rain, with the first significant snowfall occurring on December 29th. Any snowfalls previous to this were few, light and of short duration and would have disappeared quickly with the temperature and rain.
As hopingforjustice pointed out, in southern Ontario there are microclimates, so even if there is snow at one place there may be none nearby.
With respect to foliage, timing of leaf colour change and drop is dependent on a lot of things, including the previous years' climate, soil moisture, air temperature and humidity, solar radiation, species, genetics and health of the tree. Delays of more than a month from average can happen. Generally for all trees, the degree of colour change decreases and date of drop increases with higher temperatures and lower solar radiation, both of which happened in 2012 (the fall was unusually overcast, which mutes colour change).
While snow does not initiate colour change or leaf drop other than by the associated low temperature, ironically the December 29 snowfall brought the green leaves down from my holdout trees.