Creatives
You can add Creatives from the Campaign launcher very easily:
Uploading them at that moment by clicking on the 'Upload' button
Clicking on 'Add' from the Creative table, if your creative has already been uploaded.
Creatives added
Once added to the campaign, they will appear as a list on the top part of the Creatives section on the Campaign launcher, as shown on the screenshot
Filter Creatives
You will easily be able to filter by Type, Size, among your uploaded creatives by using the 'Filter' on the top left side of the screen.
Search Creatives
You can always search for creatives you've used in the past, by typing the name on the search bar on the top right of the section.