The best way to do this I would say is to import the photo, then add a new layer and paint on that above. This avoids affecting the paint which makes up the imported photo. The actual canvas in Studio / Studio Pro is separate from your photo, it's basically a collection of settings for the texture of the painting in general.
If you do this, does it give you the effect you're looking for? If not, can you let me know what isn't working for you and I may be able to make some more suggestions to help.
Resident Bug-Hunter and Support Guy