Do I have chin ptosis? How can I fix that and improve my smile? (Photo)

In the photos that you posted you don't have a ptotic chin. However, your chin is on the prominent side and this tends to be a masculine trait. Smiling, I can see your entire upper incisor which is a good thing. Most people don't show their lower teeth on "routine" smiling. In repose, you should show 3-4 mm of upper teeth. Lastly, you have a shallow mentolabial fold because of your chin prominence. Botox can help soften the mentalis muscle but I think you are a good candidate for a chin recontouring procedure to both soften the horizontal projection and decrease the width of your chin. 

Your 'upper jaw' has not grown 'down' the amount to 'show' the upper teeth (genetics); and therefore the only (and not uncommon) method to show more teeth is to have a surgery to lower the jaw. A maxillofacial surgeon can help you.The chin issue relates in part to the dental incisor positions. Note that the lower incisors are 'tilted' forward, which deemphasizes the chin prominence. This helps to create the concern you have in the area. A genioplasty and 'side of jawline' augmentation to define the jawline better would help.