Yes, you can publish both! From the Schedule A Post window, click the Attach Media button to open your media library.
From your media library you can publish images and videos you've already used in other posts, connect external Dropbox or Google Drive folders that have media in them, or upload images and videos from your computer or directly from a URL.
Once you have decided on an image or video you'd like to publish, click on it and then click Use Media. From this screen you can also see if you've already used the image or video in a previous post, and if so, how many posts and clicks are associated with it.
After clicking Use Media, you'll be taken back to the Schedule A Post window where you can continue with constructing and publishing your post!
Here is a short video covering publishing videos as well:
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