Sorry for the slow feedback,
as piggy said :
-1 Use MPQextract to get a folder and access all the files content
-2 Now finds Blp files you want to change and then use Blp viwer or the script to convert them into png and modify it on photoshop
Use the script to convert your png file to blp
-3 delete all the files you don't modify in the extracted MpQ
for exemple if you modify the Black Arakoa skin just keep [Extracted Expension (previously MPQ) / CREATURES / ARRAKOA / Arakkoa_Warrior_Black.blp] )
-4 Finally use Drop mpq to compress the folder to a MPQ archive, now you can use it as a patch (Name it Patch-#.MPQ or if it doesn't work patch-frFR-#.MPQ change the frFR to your realm country)
Any questions ?

+rep appreciated if you like (=