I might be mistaken but I believe UBB threads doesn't put any text in images, but in plain HTML.
If what you're asking for is how to use images instead of text, I think in the languages.php you can make the language items of the New Topic, Reply, etc wordings into <img src="http://www.yoursite.com/folder/newtopic.gif" alt="New Topic" /> tag (replace "newtopic" and "New Topic" with what fits, also it's not tested so maybe it's not possible). Since UBB.threads doesn't have a default image set, you can use the nice site you gave above to make your button images, although it's originally made for YaBB <img src="https://www.ubbcentral.com/boards/images/graemlins/smile.gif" alt="" /> Hope it answers your question.
I personally like Infopop, and am using UBB.classic. I read your other thread and I personally think that Infopop's support is generally very good.
