Hey @robertofd1995,
at the moment you can take screenshots while you are in the game and then press the screenshot key on the Stadia controller. If you go into the Stadia App, you can see the pictures in the bottom of the site.
Unfortunately at the moment it's not possible to share them, I don't remember atm (don't have my phone here) if it was even possible to download them, so the best way is to take a screenshot of the pictures.
Of course this is not really an easy way to do, if you want to get a bunch of pictures, so I would recommend to leave feedback in the Stadia App for the developers.
Until such time as this is an option within either the Stadia app or webpage, the alternative or at least one of many options would be to use the Google takeout tool to download your Stadia data which will include screenshots and video captures.
The link to the takeout tool is as follows: https://takeout.google.com/ you can select whatever data to download, and if you just require the Stadia data there is an option available just for that.
Not a very quick and easy approach, but one that does at least work. Once you have downloaded the data you can then share it via whatever means you like.
I don't know you, but I cannot log in using stadia only consoles and Ubisoft platform
photos save to assasns Creed website not stadia. login there to see them