Create a post in a PM or in a category like #staff, and include the desired image in the post. This will lupload the image to Discourse and make it available. In the resulting post, you can right-click the image to get the URL where it has been uploaded.
You could also create a theme component that includes the image as an asset. For that you’ll need to follow something like Include assets (e.g. images, fonts) in themes and components.