^^^
So this is the default Interface folder from the Blizzard Art Pack, downloaded from Blizzards website directly. I have already converted the BLP's to PNG which you can see beside the original files. (PS I noticed the small black box for the preview which is different from the rest... weird windows glitch - I hit f5 and it looked normal again!)
^^^
These are the PNG files as they have been edited. They are only 1 layer and apart from being a different color they are exactly the same dimensions.
^^^
Here you can see the modified PNG's next to the freshly converted BLP's.
^^^
And finally you can see the BLP's in the WoW Folder (note the file path in the bar and names of folders).
Like I said, I have modified menu bars, buttons, player frames etc this way and it has worked just fine. With the load bar... nothing