Hi Morfy,
Thanks for asking! I understand the confusion – Reference layers can not be edited. You need to add a new layer on top and draw on that instead.
Imported images are bitmap/raster data. Animation Paper has a special line engine for your drawings, so they are technically not compatible. Try lassoing a part of your drawn lines and scale it up. When you stamp it down you’ll see no degradation or blurring of your lines. 🙂
Cheers,
Niels