Display image conditionally

is there a way to display an “account creation” image only if the connected user has not completed the account creation form?

Use the formula to compute a column that only has the image URL if the user hasn’t filled the form yet. Remember that most components won’t show if their connected column is empty.

How to refer to the connected user in my home page ?

Your home page could be a details page with per user data. In the spreadsheet, you could dynamically fill the home tab sheet using Unique range from either the App: Logins sheet or your whitelist sheet. Then, when they sign in, they will be directed to the details of the row that matches their email address. In those rows are where you could add the formula to show the image or not.