How do i add a picture as a face on a character
Make a plane, scale it down, then add a picture on it after importing (Drag and drop to assets) to the plane...
If you want it 3d, you can make a cube and do the same...
Like Justin Warner said. I would like to add that you need to create a material with the image on it to add to an object. You should check basic Unity tutorials.
What should I do if the picture should be a Button? Instead of the regular button images?