Assets you need to provide
Favicon
This is the tiny icon that appears in the browser tab for your website.
If you want to use a favicon, replace the files static/Logo.png (a 32x32px PNG file) and static/Logo.svg (an SVG file of the same thing) with the icon you want to show.
There’s lots of online tools to help with this.
If you don’t want to use a favicon, delete static/Logo.png and static/Logo.svg, and remove the lines referencing them from src/app.html.
Be prepared to tweak src/lib/Navbar.svelte too, since this displays the SVG version of the logo at desktop sizes.
CMS welcome image
If you use the CMS, you can supply an image to be shown on the login page. It needs to be 300x125px in PNG format. Put this in static/CMSLogo.png.
If you don’t care about having a custom logo there, delete the existing static/CMSLogo.png, and the line referencing logo_url from static/admin/config.yml.
Add new assets for content
If you want to put pictures in your blog posts or other pages, put them in the static folder, and reference them like <img src="/myNewImage.jpg" alt="A very descriptive description"/> in your HTML or  in markdown.
Next steps
Maybe you want to go on to change the fonts and colours, deploy your fancy new site or configure the CMS.