IN TOOLS SECTION OF THE HIVEWORKSHOP there is a program called war3modeleditor by magos. get that, open the program and click file open in the top left of the toolbar of the program. once your picture is open click on edit and open the texture manager, right click and select export, type in the name of the picture and then type in ".blp" and save it in a folder of your choosing. TADA
NOTE: jpg images do not save alpha channels so you will not see any team colors. if you want alpha channels then export as a tga and edit in PAINT.NET also found in tools section and then transfer to blp.
NOTE: Magos' war3modelsditor is also amazing for model editing