Everything on Design & Content › Design in the admin that you can happily leave alone. Each of these has a sensible fallback, so fill them in only when you want something specific. Click Save Design Settings when you're done.
1242×2688 JPG or PNG, portrait. Sits behind every screen in the app. Leave it blank and fans get a gradient derived from your Primary Color — you can see exactly what that looks like in the Live Preview on the right of the page.

The fallbacks shown when a member, event or venue has no image of its own.
200×200 square, JPG or PNG. The avatar fallback in leaderboards and the activity stream. Always displayed as a circle, so keep the subject centered.

750×375 (2:1), JPG or PNG. Shown wherever an event has no image of its own — cards, lotteries and the stream.

750×375 (2:1), JPG or PNG. Shown when a venue has no photo.

This section is collapsed, and deliberately so — both settings inside it are derived from your brand colors automatically, and most programs never touch them. Expand it only if something specific needs its own color.
Map Pin Color — the pins on the venue map. Uses your Primary Color when not overridden.
Story Background — the backdrop of text-only Stories. Uses your Primary Color when not overridden.

Found in the Email section. It backs the email header when the banner image is missing or blocked by the recipient's mail client. Uses your Highlight Color when not overridden.
Tucked behind an advanced toggle in the Desktop Welcome section. Paste full HTML here and it completely replaces the standard welcome screen on desktop and mobile web. Your HTML is responsible for being responsive and for including links to log in and join. Leave it blank to use the standard welcome.