How to use modified BLP
 

I want to change the greyed out Icons for outleveled quests.

Extracted the objecticonsatlas.blp with CascView, converted it to PNG with BLPNG Converter, modified it with GIMP, converted it back to BLP and placed it in retail/interface/minimap.

Same procedure I did a couple of times, but last time is a while ago. I think I did everything the same way but the updated file is not used in the game. Did I forget something?

Edit: Instead of using the file in the reatil/interface/minimap folder I got the Derangement MinimapBlips Addon from Curseforge, converted the PNG to TGA and used it in the Addon. I also learned that there is a committed User named Matjtonn who updates the file regularly.