Hello and thanks for the question. 
The asymmetry you mention is pretty minor and well within the spectrum of what would be considered normal. It looks like you have an athletic frame and having an implant placed partially behind the pectoralis major muscle would be a good idea. This is often called a dual-plane procedure and is one of the most common techniques. There can be a fairly visible 'step-off' effect if it is placed behind your breast alone with your physique.
Are you planning on having any more children? If you are it might be a good idea to put things off until you've finished. You are fortunate in that the position of the nipple relative to the crease under the breast (that we surgeons call the IMF-Infra Mammary Fold) is in a good. This means you could easily have a straight forward breast augmentation without having a lift at the same time.
Try and pick someone that has a Vectra Machine as it's a really accurate planning and communication tool for someone with your physique and build. Broadly speaking most people usually want a fuller or more enhanced look, a natural or athletic look and the restoration of the breasts they used to have before children. Look through some galleries to get an idea of what things you like but bear in mind that everyone is different and your surgeon should treat you like an individual to customise the best treatment to you. 
At Park Clinic Plastic Surgery we have developed an essentials video series for anyone considering having surgery that covers a lot of the most common FAQ's. You can find it here:
http://www.parkclinicsydney.com/plastic-surgery/breast/breast-augmentation/

Hope this helps. If you want to reach out directly you can get me at dralex@parkclinicsydney.com
Kindest regards,
A