Under Organizer → Sponsors you manage your organization's sponsors. Once created, you can display them on your events.

The list includes a search box, and for each sponsor you can edit or delete it (with confirmation; it cannot be undone).

Sponsors are selected per event under Settings → Event information, in the sponsors section. That way you can show different sponsors on each event.

Use logos with a transparent background (PNG) and similar proportions for a cleaner look on the event website.