From the layers
Tap on the import icon located at the top of the layers pile to access the import options: Photo Album, Files and Sequence.
Photo Album
Import images from your Photos app.
When you import an image in a shot, the image will automatically resize to the shot’s dimensions.
Files
Allows you to import images and videos from your Files app.
If you import a PSD, it will import the file with the different layers, however it will not include folder structure or blending modes.
Sequence
The sequence import allows you to choose the folder(s) containing your image sequence. Importing several folders at once will create a layer for each folder.
To import a video, tap on the icon at the top-left of the layers pile, which opens the layer types panel. Choose the video icon, which will create a new video layer. Then tap on the import icon on the layer to add a video clip. You can import several video clips in the same layer.
To import audio, tap on the icon at the top-left of the layers pile, which opens the layer types panel. Choose the audio icon, which will create a new audio layer. Then tap on the import icon on the layer to add an audio clip. You can import several audio clips in the same layer.
From the Timeline
It is possible to import an image or a photograph from the timeline.
To do so, you need to select an image with a double-tap to display the action panel at the top of the timeline.
Tap on the Import icon and choose between 3 possibilities: take a Photograph, open the Photo Album or open the Files. 
DRag & Drop
You can also drag and drop images using the split view.
To do so, open the iPad dock by dragging it up from the bottom of the screen, then drag the app you want to use, like Files, to the right-side or the left-side of the screen. When both the apps are displayed on screen, drag the image you want to the Callipeg canvas, and tap on the transformation icon or another tool (selection, brush, eraser) to validate. Going to the next frame will automatically validate and you can continue to drag and drop other images.
