Continuing the discussion from Features Wish List:
Annotating / editing images and PDFs directly in the app is on our todo list. The plan would be to eventually support at least the standard annotation tools of macOS or iOS. Note that on the Mac, you can simply double-click the image or PDF, and it will open in the application of your choice, where you can edit it. When you save, the document from your library will actually be changed, as expected, and you can then close it.