Built-in alert and action types have been using stock EUI icons, but pagerduty and logos for some planned action types are not included in EUI. EUI does support custom icons, see "custom SVGs" in EUI icon docs

When a custom icons is needed, where should these resources be located and how are they specified when registering actions or alerts in the UI?

Ideally a 3rd party can include these resources with their plugin containing their alert types and action types.