Steps

• Setup JIRA with an incoming create or comment email handler.
• Set up a user with an email address and change its profile so they are notified of their own changes.
• Login as that user.
• Load into jira some custom icon and attach it to priority
• Create an issue that has priority with custom icon in it.
• Check that user's inbox in Thunderbird/MS Outlook.
• You should see a issue creation notification email and the new priority icon should be in it.
• Reply to this email so that a comment gets added to the JIRA issue.
• Check the JIRA issue in the browser.
• [BUG] A new comment will be added to the issue, and the priority icon will be added as an attachment to it.

Root Cause

This is caused because custom images were not tagged with the png metadata that is necessary for any email image sent from JIRA is meant for inclusion as an embedded image in email messages.
possible solutions include:

• try to tag images during configuration step
• change way how images are uploaded for priorities to avatar uploader