1. If you want to use an in game texture open the Warcraft 3 Model Editor, go to: Windows> MPQ Browser> File> War3.mpq, then go to Textures folder, find a texture you like and remember the name of it. Then open the card model, again open the MPQ Browser, find your texture and then right click it (be careful not to double click it) and click "Use as Texture" option.
If you want to use a custom texture, open the card model and go to: Windows> Texture Manager, then right click anywhere on that window and "Import" the custom texture.
2. Go to: Windows> Material Manager, open the first Material(there are 4 of them), then open the third Material Layer(there are 3 of them), change the "Texture ID" to the path of your new texture. Click OK, OK.
3. Go to: Windows> Animation Controller, and play the "Stand" animation, to see how it looks now.
4. Go to: Windows> Texture Manager, right click "UI\Glues\Loading\Backgrounds\Campaigns\HumanSymbol.blp" and "Remove" it.
5. Close all windows and Save the model.
This is the most simple way to do this. If you need additional details, or If you have any questions, just ask. You might want to know how to use a part of a texture, not the whole image.