I'd go for a combination of Flash and Flex myself, but I already own both. You can use Flash to export an asset library and then build the game as an ActionScript project, which you can compile using the free Flex compiler.
If your assets don't need any timeline based animation, then just go down the Flex SDK route as you'll be able to embed those assets into your project with pure ActionScript.
I wouldn't recommend going down the Flex framework route so don't code it in MXML. You don't need the extra weight of the framework for something like this, an ActionScript project will do just fine.
Also note that the next Flash Player has support for hardware acceleration and you can already download a version of the Flex compiler to target this new version. I guess that choice would also depend on how CPU intensive your game is going to be, when it would be released and what group of people you'd be targeting.
I wouldn't recommend going down the Flex framework route so don't code it in MXML. You don't need the extra weight of the framework for something like this, an ActionScript project will do just fine. Also note that the next Flash Player has support for hardware acceleration and you can already download a version of the Flex compiler to target this new version. I guess that choice would also depend on how CPU intensive your game is going to be, when it would be released and what group of people you'd be targeting.