Open up an issue and tap on the image icon to the right of the photo preview area. On the pop-up menu that appears, tap on "iCloud/Dropbox". This will open up a new screen showing your iCloud Drive images. If you wish to access your Dropbox instead, tap on "Locations" in the top right and then tap on "More...". You'll find Dropbox located here.
You can tap it once to open it straight away. If you wish to keep it in your list of locations for easy access, tap the switch to the right of it. If you have other similar services, such as OneDrive, you'll find these listed and available to add too.