That version of the graphics pack there is I think pretty old and out of date now, because Ironhand updated the version in this thread beyond your game version.
Unfortunately, I am not sure if it would be possible to find a version that matches your exact Dwarf Fortress game (you may find one floating around somewhere - if you do, you could selectively install certain files - ask here if you do).edit: okay Gunnarr you're in luck, I have I think reconstituted the version of Ironhand's files that he released for 31.25!
The only version I could find was in fact contained inside an April Fools release he did that made everything into llamas but don't worry, I found a separate 31.25 version of his graphics that are not in fact llamas, and I believe he didn't go to the lengths of changing anything else! Probably!
edit2: I also added in his dwarves from later versions as I believe they will display fine (if it was you that downloaded it before I edited this, please redownload!).
Here is the file.1. Backup your existing Dwarf Fortress install folder.
2. Extract everything in this zip directly into that folder (you should be asked if you want to overwrite files, if you do not get this message then you didn't extract it directly over the top).
3. Now, in order for your save game to show these new graphics, you will need to copy the raw folder in this zip on top of the raw folder inside your save game folder. This folder will be in data\save\region#\raw - where region# is the correct folder for your save game, it tells you this on the 'Continue Playing' screen (you can however simply copy the raw in the zip into every existing region if you don't know which one it is).
Enjoy, and I hope it works