Problem

As can be seen in the below screenshot the avatar is missing on mail conversation:

Diagnosis

Make sure it is system avatar

If not using any custom avatar there won't be any avatar folder in this path <jira-home>/data/avatars/ 

Also you can check the avatar in JIRA itself.

Cause

Gravatars are not enabled. 

To use a Gravatar:

If Gravatar has been enabled, your Gravatar (i.e. the Gravatar associated with the email address in your user profile) will be shown along with the JIRA built-in avatars and any avatars that you have previously uploaded. To change your Gravatar, log in to Gravatar.com and follow the instructions on that site.

Resolution

Enabling Gravatar support:

1. Click the  icon in the JIRA header and choose System.
2. Click General Configuration in the left navigation panel.
3. Click Edit Settings.
4. Select the Enable Gravatar Support.
5. Check Gravatar ON or enter the URL of your Gravatar server in the URL field, or leave as default if you wish to use the default Gravatar service
6. Click Update.