Well, in the model the image is of a certain filetype. Maybe u imported the model's skin with other image file type than the one used in the actual model.
To figure it out, use Magos Model Editor.
You open the model, go to
Windows > Textures Manager
From there you see which skins are imported in the model. If there is not the one you actually uploaded to the warcraft editor, then do this :
Go again at the -> Textures Manager -> Right click and import the skin you uploaded on the wc3 editor to the Magos Model Editor -> Go to Materials Manager -> From there , you must search the materials (1 by 1) until you find the right one and change the skin.
If u want more info, pm me.