With this guide i will show you how to make a custom loading screen, you will see its pretty easy to do. First u will need some tools and those tools are:
My Warcraft Studio – This is the main tool wit this tool you will create the .MPQ archive/file and edit/open already existing .MPQ archives.
BLPConverter – This tool converts the .png file to .blp file.
And preferably u will need some image editing program. Personally im using PS 4.
You can download MWS and BLPConverter from:
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OK when u have downloaded that we can start working.
First start MWS and go to FileOpen MPQ Archive
From there go to your
WoW Data folder and open enGB or enUS

Once you are there open locale-enUS.MPQ (in my case).
Go to Interface > GLUES > LoadingScreens

Now find the screen you want to change and right click on it.
Select Copy Treepath and then open some text editor like Notepad or Word
and paste the Treepath there, you will need it later.
Now edit your picture (the 1 that you want to use).
When you are done with the editing you will have to resize it
to either 512x512 or 1024x1024 it depends what screen you are
replacing. You can see what size you need when u click on the screen
in MWS.
ITS IMPORTAN THAT THE IMAGE THAT YOU ARE EDITING IS THE SAME SIZE
LIKE THE ORIGINAL SCREEN IF ITS NOT YOU WILL GET A GREEN SCREEN
AND YOUR WoW WILL CRASH WHEN IT TRYS TO LOAD THAT SCREEN.
Save the image you made as a .png image.
Now drag that image to BLPConverter and let him do his job.
When the image file is converted to .BLP file we can go on with
our work. Go to MWS and select Pack and then Create MPQ Archive.
Now go to Pack again and select Add file to archive

When you find your file don’t press OK on the box yet. Remember that Treepath
that we pasted in the notepad well copy it again and paste it in the area
where it sais Input the pathname of the file to be.
Now go to Pack and select Save and close archive.
A FEW WARNINGS:
1.When you save the .MPQ file remember to name it patch-L.MPQ (EXAMPEL) small p in patch BIG letter after – (It can be just 1 letter) and BIG MPQ after .
2.And remember to save the .MPQ file inside the Data folder, not the enGB or the enUS folder when u want to use your .MPQ file
I hope my guide helps you. You can also download it in Word format with the images from:
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Im sorry for the big images.