thumbnails ?

Hello there!!!

so i started using wordpress yesterday and so far its great all setup nice)) but now when i want to make a new post i dont know how to insert the thumbnail picture
i am using this template: https://farm4.static.flickr.com/3286/3109388257_a33960eaf3_o.jpg

and i need to know how to insert a pic (like the walli-e one)

thanks for help in advance

oh & sorry about my bad english :”>