I'm a 34D wanting 300cc implants. Should I go smaller as I already have breast tissue? (Photo)

Thank you for the question and picture. Based on your photograph, I wonder if you will be best off undergoing breast lifting, along with breast augmentation surgery. Ultimately, careful communication of your goals (in my practice I prefer the use of goal pictures, direct examination/communication in front of a full-length mirror, and computer imaging) will be critical.  
Generally speaking, the best online advice I can give to ladies who are considering breast augmentation/lifting surgery ( regarding breast implant size/profile selection) is:
1. Concentrate on choosing your plastic surgeon carefully. Concentrate on appropriate training, certification, and the ability of the plastic surgeon to achieve the results you are looking for. Ask to see lots of examples of his/her work.
2. Have a full discussion and communication regarding your desired goals with your plastic surgeon. This communication will be critical in determining breast implant size/type/profile will most likely help achieve your goals.
In my practice, the use of photographs of “goal” pictures (and breasts that are too big or too small) is very helpful. I have found that the use of words such as “natural” or "D or DD cup” etc means different things to different people and therefore prove unhelpful.
Also, as you know, cup size varies depending on him who makes the bra; therefore, discussing desired cup size may also be inaccurate. Again, the use of computer imaging has been very helpful during the communication process, in our practice.
3. Once you feel you have communicated your goals clearly, allow your plastic surgeon to use his/her years of experience/judgment to choose the breast implant size/profile that will best meet your goals. Again, in my practice, this decision is usually made during surgery, after the use of temporary intraoperative sizers.
I hope this (and the attached link, dedicated to breast augmentation/lifting surgery concerns) helps. Best wishes.

breast size is a very personal issue. What is the correct size for you is based on your tissues first of all and then on what you feel comfortable with. Your plastic surgeon is there to help you make these decisions.

I was looking at the photograph you provided.  You are 5'7'' and 60Kg, presently a 34D.  my thoughts are you are large enough.  Given you height, approximately 350-400cc's of volume will take you up one full cup size to a DD.  300cc of volume between a D and DD. Your concern seems to be more about shape.  I would consider the newest shaped silicone implants which come in differing vertical heights as well but these implants require a seasoned aesthetic breast surgeon as this implants are less forgiving requiring careful measuring and pocket creation.  I also noticed you have some asymmetries of shape and size that require careful measuring to confirm on a physical examination, again, with a board certified surgeon.