Hi Asmaa – thanks for your question!
Yes, imported images are meant to mainly be used for reference purposes. They go into a “reference layer” – which basically means that they are not editable – they are for drawing on top of, like rough animation from other software, or video, or rendered 3D for moving backgrounds or for rotoscoping, etc, etc.
We are working on an interface and features, to make you able to move, scale and rotate complete layers, so this will be possible for reference layers in one of the following releases.
But drawing in imported images will not be possible. You see, Animation Paper has a special line engine that gives you many great advantages, such as being able to scale and rotate your drawings without any degradation or blurring. If you haven’t noticed, try to lasso a little doodle and scale it way up. It is almost magical 😉 But this means that standard bitmap images imported from other software does not have the ability to do all these clever things. Best practice is to do both your rough work and your clean-up/inking  within Animation Paper.
What you CAN do with reference layers are edit them timing wise – i.e. set their exposure by dragging out clones, move them around, cut, copy and paste them – on a whole drawing/frame basis. So, you can do your timing and sync to your drawn animation and audio track if you have that.
I hope this makes sense. Please let me know if you had anything else in mind.
Thanks,
Niels