Can I add welcome slides and a branded login screen to my app?

Yes. Both are under Settings → App Settings.

Welcome Screens are the onboarding slides shown when the app is first opened.

1. Click Add slide and fill in a Title, optional Subtitle and an Image. You can add an optional Button label and Button target (a screen or URL).
2. Set Status to Show or Hide, save, and use ↑ ↓ to set the order.
3. The Show welcome screens switch turns onboarding off completely, so the app opens straight to Home.

Slides marked Default can be edited but not deleted.

Login Screens holds the images shown on the app's login screen. Click Add image to pick one from your media library. To remove one, hover over it and click Remove.