Feature Image for Portfolio Projects in "Untitle"

First of all, congratulations for this amazing Theme. I tried a lot and this is with difference the most elegant and modern of all (in all the devices)
I have just an issue: I’m working in a blog in wordpress.com and I don’t know why in this theme it’s not possible to publish an Feature Image for Portfolio’s Projects, only in post and pages.
I know that it’s compatible because I have some Featured Images in Projects I’ve created in other themes I tried before, and this theme (Untitled) show them when a page is in [porfolio] mode. But I can’t add any new ones.

I would appreciate any help because I don’t have programming skills and I love this theme.

Thanks!!