How to pull image out of post (source is Flickr)

Hi, I’m thinking of changing the theme of my blog and I have a question.

I’ve installed Pinboard theme on InstantWP and now I’m getting familiar with the theme. I’ve imported all of my posts (around 250), but I can’t pull out photos out of my posts – to be seen on the homepage as featured photo. In my blog posts, the majority of photos were not uploaded on the server, but are linked to my Flickr profile.

I don’t know how to choose those (Flickr) photos in the Set Featured Image. Do you have any idea how to select those photos, because it seems that for featured photos pinboard only uses photos that were uploaded to the server.