It sounds like you're in the image-only mode. To get out of that so that you can see your drawing and the image at the same time, follow these steps.
- Tap on the bottom of the screen while you’re in a drawing.
- Tap the settings button.
- Drag the "Lines" slider down to about halfway.
- Tap the icon that shows 3 bars in a circle.
- Drag the "Image" slider down to about halfway.
You should be able to see both the lines of the image and what you're currently drawing from the image now.