I can actually notice a bit of changes, but it's very hard to see...
Here is what u do:
Open your Wc3 folder eg.:
Then make a folder that has the same path of the BloodMages skin eg.: (can't remember the path)
Code:
c:\games\wc3folder\units\human\herobloodelfmage\
Now, you put your blp file there, with the same name as the normal bloodmages blp file, again eg.:
Code:
c:\games\wc3folder\units\human\herobloodelfmage\herobloodelfmage.blp
Now you open your Viewer and get the bloodmages.mdx file and your skin will be applied to that model.
To take a screenshot, either use the prt.sc on your keyboard, or even better yet, find the snapshot (it's a camera) button in Viewer and take the shot.
Now, post some real screens :)