When I was coding for iPhone with Xcode or even in Gamesalad I had my artist make all artwork to the power of 2, so it could be upscaled. However even when we was making for a 960x640 or even older 320x480 all of our vector work was created at much higher scales. This allowed you to easy put a build out for 4s, 5 and iPad without too much work.
In C2 I have been testing (and I am a noob at c2) but it has familiar elements that I have grasped rather fast. I have been looking at the resolution and aspect ratio. So a game made at 1280x720 is native 720p and will look perfect on all screen in the 16:9 ratio but letterbox on a 4:3 even at high resolutions like 1680x1050.
So targeting a aspect ratio is easier then a set resolution, and the stance we have taken is most devices be it smartphones or tablets adopt funny resolutions but tend to be 16:9 widescreen ratio.
A simple test sheet would be to draw a background image with the corners highlighted and test it across multiple devices and browsers. This will show you if the image fits and if the aspect ratio is correct and does not distort the image. Don't forget that just because it fits it may not look correct.
I hope that helps a little. If you require ill post a test .capx that will assist you as discussed.