There is no absolute consensus but my opinion is that all cosmetic breast augmentation implants should be below the pectoralis muscle and done through an inframammary crease incision. The proper size of the implant (for round, smooth surface silicone or saline filled implants) is one that has a diameter that matches the internal width of you breast (about 3/4 to 1 cm less than the external width) and has a low, medium, or high profile depending on whether you want to increase your apparent size by 1, 1 1/2, or 2 cup sizes. Nipples and areolas are a separate issue and are not affected by an augmentation of implants that are properly fitted and positioned.
Equal looking results can be achieved with silicone gel-filled or saline-filled implants and saline-filled implants have several advantages over gels but silicone gel-filled implants feel like tissue and saline does not. In your case, with thin looking coverage over the lower pole of the breast, you might want to consider a silicone gel-filled implant for this.
Based on your photos, I would recommend silicone implants placed under the muscle. This will lower your chances of rippling. As for size, I find it useful for patients to try on implant sizers in the office, just to get a general idea. I also highly recommend consultation from a board certified plastic surgeon.
Thank you for your questions and photos! I'm not exactly sure what you mean by your first question so will need clarification on that one. My personal preference are silicone implants for most patients because I think they provide a softer, more natural appearance to the augmentation, especially for women with little native breast tissue. Similarly, I prefer under the muscle for most patients for a few reasons: 1. Again I think it provides an added layer of tissue between the implant and your skin to soften the implant edges and make it more natural. And unless you are a body builder and lift heavy weights, the chance of a significant animation deformity is fairly low. 2. Our literature shows the risk of capsular contracture (bad, hard scars around the implant) to be lower when the implant is placed below the muscle and through an inframammary incision (fold beneath the breast). As far as the sagging, you very well may need a lift at the same time as your augmentation. That and the size of the implant really needs to be assessed at the time of your in-person consultation to give you a good estimate. It's great to have a goal photo to give your surgeon an idea as to visually what you want. Implant selection is based on measurements taken during your consultation as well as some other factors that are best assessed in person. I would guess somewhere around 400cc to achieve your goal but that is ONLY an estimate. Make sure you choose a board certified/eligible plastic surgeon! Good luck!