It sounds like you're in 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.