Site Builder > Pages > Page Settings Icon found to the right of each page allows you edit the title of the page which will update how its shown in the menu.
You'll also have the option to adjust its URL. Enable or disable the page or add or remove the page from the header or footer.
Clicking Edit Page or on the page title itself will take you into the edit mode of that particular page, where you can edit existing sections and content or add new sections and content using the Add Content and Features link within an existing or newly added section.
Text & Images
Enter your text and images. Choose 1, 2, or 3 column layout.
Section title
Use this block to add a formatted section title that defines the content.
Divider
Clearly separate content using a divider line.
Banners
Select one of our banner types to add a banner to your page.
Photo Albums
Add a photo album to your site.
Video
Add a video from YouTube, Vimeo or other video sites.
Events
Add event information or a countdown for your event.
Event countdown
Add a countdown to your event. Displays the date and time of event with a countdown to the event.
Event information
Add the details of your event. Displays the title, description, date, time and location of the event. A map is displayed for events with a physical address listed.
Tickets & registration
Add a form to sell tickets, accept registrations, and display who's coming.
Sell all tickets and registrations
Display all tickets and registration options.
Sell one or many specific tickets or registration option
Display only one or many specific tickets or registration option at a time.
Who's coming for all tickets and registrations
Display names of people who registered or purchased any ticket on your site.
Who's coming for a specific ticket
Display names of people who have registered or purchased one specific ticket.
Donations
Add a donation form or show the donor list.
Custom forms
Add a custom form to your site.
Contact form
Allow users to contact you via a form on the page. Displays a "Contact" form.
Social media sharing
Allow users to share the page on social media. Displays sharing icons for Facebook, Twitter, and LinkedIn.
Updates
Add updates to keep your visitors informed.
Guest Book
Visitors can post their comments in a Guest Book that displays on the website.
Facebook comments
Add Facebook comments to your site.
The image should optimally be 1920 x 480 pixels. You could easily find wallpapers on Google that you can then crop to fit as the desktop banner. Then using something such as Paint3D you can select the crop option to size it down.
Mobile banner is a square 768 x 768 pixels. You could always crop the desktop banner into a square image to work as the mobile banner. 768x768 is optimal but as long as its a square image it will work for most cases. Here's the desktop banner cropped down to a rough square 420 x 385.
If your on mobile you'll see the mobile banner above. Otherwise you'll have the placeholder desktop banner since its not set for this example.