Have you checked if the files are not in a subdirectory or a in directory above the interface directory?
Check the address bar! For example, you should have the following file (assuming you are using windows)
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C:\Program Files\World of Warcraft\Interface\AchievementFrame\UI-Achievement-AchievementBackground.blp.
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And not one of the following:
C:\Program Files\World of Warcraft\Interface\AI-Art\Interface\AchievementFrame\UI-Achievement-AchievementBackground.blp.
C:\Program Files\World of Warcraft\Interface\Interface\AchievementFrame\UI-Achievement-AchievementBackground.blp
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If the achievement frame has the default looks of the game, you extracted in the wrong place.