Age verification popup
You can create a popup module in Journal > Modules > Popup. into which you add your custom image, such as this one.
In the Popup module General section turn off the Close Button and add your custom image in the Popup's content section. Without a close button, the popup can only be closed from one of the buttons from the popup footer (the one set to affirm the age, more info below).
In the Popup's footer, you activate the "Don't show again" option and have it checked by default (so the popup only shows up once).
The first button can be set to affirm "YES" and have the link set to None. Without a link, clicking on the button will just close the popup, while also affirming the age.
The other button will say "NO" (add have the link set to any other website). You can add custom styles to each button if desired.
Finally, add the Popup module to your pages in Journal > Layouts > Home > Special Modules - All Layouts (top right corner button, see point 2a in our layouts docs here).