There is entirely no issues with posting comments on photos to our knowledge looking at the software. Remember if your using vbulletin try disabling the plugin system before reporting issues because you may have a hack conflict. Also make sure your software has not been modified in any way.
If your not the user whom posted the photo you should see either a quick reply box or a full comments box when you hit post comment. That is of course if permissions to post comments have been set.
At any rate we would need you to provide a url with details here to support and show your claim as well as a test login.