You can either create these in a graphics software if you have some graphics knowledge or download specific images from the internet. However if you download you must look carefully for images that do not have watermark on them. You can get someone to do then for you using Photoshop.
Or try searching for game templates, I got this idea here techyv.com/questions/i-need-joomla-gaming-template-my-new-cop-game. Someone is looking for a template to make a cop game. You can try to find a template for farmville.