One of the main reasons for the migration was switching to a theme-based interface, which allows for a more granular customizations.
Basically, just a single setting which controls if to show or hide welcome banner was moved per theme setting. All the rest settings are organized in a single section Admin > Appearance > Interface & layout.
It looks like the image has the height larger than the size of welcome banner, so it is vertically centered. If tweaking CSS is a challenge, another option is to upload image with a cropped height fitting into welcome banner.
If you are referring to the text “Please sign up for…“ below the header, it can be edited in Admin > Appearance > Site texts, search for js.welcome_banner.subheader:
