Seems this is a bit tricky as SharePoint uses the modified by field for the poster.
There's two ways I could see this working. I don't know XSL well enough to advise you on creating an XSL template and injecting it into the web part, but seeing as this is an xslt web part you presumably could do this and return whatever value you want in any area. You may be able to get hold of the threaded template as a starting point.
-script on the page will run when the page is fully loaded (since this is client-side).
-The script will look through each post and extract the "ID" of each item, as each post is an item.
-The script will then query your discussion list to find the name of the person who created that item.
-The script will then query the user profile service and populate the image of that user on the left hand side.
I hope that made sense. Happy to clarify if that was a bit convoluted.
Have you looked at this forum thread?
It shows you where the original thread.xsl is stored on your SP server and you can create a new one and reference it in the XSLLink property of your web part. In your new xsl file you can return whatever value you want for the username on the left hand side. I have not tried this myself but the word is this seems to work.
I don't need more customizations. Even we use the default thread view with content approval, the posted By will change to the approver's name.
So briefly i need an approval process and also to display the replied users name for a discussion thread.