The original staff photos were taken by Peter Anderson, whose website is linked here.
I used Adobe Photoshop to create a winter weather effect, making it seem as if the photos were taken in snowy weather, rather than in the middle of the Summer. This was done by creating a new black & white and brighter layer and overlaying it over the original. From there, I tweaked where this "snow" overlay appeared as well as further adjustments. Finally, I added some fake snowflakes over the entire image to complete the effect.
It's far from a perfect transformation from Summer to Winter, but it is enough to create an approximation of another season.