Probably just about everybody on here already knows this, but for newbies like me, perhaps this will be of some benefit.
I found that sometimes when I export from AR to an image file, I get some unwanted (for lack of a better word) rough edges where there is a definite color change.
What has worked for me the last few times I've tried it, is to just right click on the painting in AR, so that you get a full screen view, then do a screen capture by hitting "print screen" (not sure how Mac users would do the same).
Then open up the graphics program of your choice, paste, and you should have a clean image.
Hope this helps someone.