You can keep the images, but you'll have to rename them to what they are called in phpbb3. Then, you'll have to edit the imageset.cfg to change the image sizes, because lots of them are different in phpbb3 and if you don't change it, they stretch the graphics SO BAD!
Here is my post on subBlack 2.0 for phpbb3. I didn't make any new images, I just editted the stylesheet and config files. But you might be able to make the remaining required images by your self, or get the maker to do it, then you can make some changes to it to make it look like your forums (or atleast if the link you put that was marked [spam] you put was the same thing as in your signature)
View the last post, it's the best screenshot of it.
viewtopic.php?f=26&t=24687
It is currently not availible for download publicly, but I could send you a PM with the link if you want.
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