BP Social Article images

I have installed the latest BP-Social theme but cannot understand how the homepage 'article' images are controlled.

I've found gallery.php doesn't contain the image resize. I get the original image I upload for the post 2000x3000 resized by 'something' to 340px.

My question is: How can I change the size of image called (I'd rather call the medium size image) and how are these images being resized on the home page.

Thanks