Using an existing Library item

Once uploading the image to your art library, you can bring it up via the image search within Roll20. From there you can drag it into a handout, a character journal portrait, card decks, rollable tables or to the tabletop. As for the campaign icon, as of right now, that has to be uploaded outside the application, so you'll end up having to upload it to your art library twice since it's already uploaded from within the campaign itself. My recommendation is that whatever art you want to use as your campaign icon, assign it as the campaign icon before uploading it inside the actual app. That way it's only uploaded once and it'll be automatically searchable within your campaign when you load it.

Glad to help, Phillip!