Open a canvas and tap the Image icon in the toolbar.
That will launch a menu of import locations.
After you import your image you can either:
Add a drawing layer above it and draw on that
Or you can launch the Layer Actions menu and choose Convert to Pixel Layer to treat the layer as a drawing layer (to launch the Layer Actions menu, tap the Image Layer thumbnail so it's outlined in blue, then tap it again)
Is that what you were hoping to be able to do? Let me know if it's not. And let me know if you're having any trouble.