speakers for my new ride pics

Hello, and congrats on the new car!  It's very beautiful indeed!  I am an owner of a 2009 Mazdaspeed 3 and have upgraded the entire audio system.  I'd first like to say that since this is a new year model, the aftermarket support for it is yet to produce aftermarket install kits for stereo replacements, etc...  However, since you're not interested in upgrading the stereo itself, you're in luck.  Firstly, I'd like to point you to a few Mazda 3 specific forums which will have more information pertaining to your model specifically:

www.mazda3forums.com

www.mazdaspeedforums.org

Those two site both have 2010 model specific forum listings, and you'll be able to find more on specific speaker sizes and wire locations and colors there.

It is definately possible to upgrade your sound by replacing only the factory mids/highs and you'll notice a huge improvement over stock.  If you're wanting to add a sub and amp, a line output converter (LOC) will get you the signal you need to feed your amplifier, or if your amplifier has high level inputs, you'll be able to just tap into the existing speaker outputs of the headunit and run them to the amp.  To take it to the next level and help get even better audio, look at solutions such as Rockford Fosgates 3Sixty.2, JL Audio's CleanSweep, and I'm sure there are other systems out there like these.  What they do is level out the sound produced by your factory headunit and give you pre-outputs for your amplifiers to provide them with an even, clean audio signal.  Most factory stereos no and days do things like limit the amount of bass being fed to your mids/highs a higher volumes to prevent distortion.  These systems, when properly installed, can prevent this from happening and maintaining a true audio image.  Do some research and check out the forums listed above for more info specific to your new vehicle!