How to display a author picture in comment form?
I don't think you need to do coding for your requirement. You can take help from this documentation for same - https://drupal.org/node/22271. It says for D7 :

Enable user pictures for Drupal 7


Navigate to Configuration > People > Account settings.
In the Personalization settings, select Enable user pictures. 
For Picture directory type pictures
Under Picture display style, select thumbnail (you can change the size settings
Click "Save configuration".


Next, ensure your user pictures display in your theme. These are enabled by default in Bartik, the default theme of Drupal 7 , so you may have to check this with any new theme you add. 


Go to Appearance and click "Settings" for the theme you have enabled as default. 
In the Toggle display settings, select User pictures in posts and/or User pictures in comments.
Click "Save configuration".


If your theme doesn't support user pictures, then you should look for bartik &amp; apply same structure for TPL.