If you're running iOS 11 on your iPad, you can drag an image from a GoodNotes notebook page to another app that also supports drag & drop.
1. Tap and hold on the image
2. Tap "Edit" in the contextual menu
3. Press and hold on the image again until it "lifts" of the page
4. You can now drag the image to another app
Drag and drop does not work with images in PDF documents in GoodNotes or with images that have been added as an individual page.