One way to find free images of stacks of money online is use free stock photo sites such as morgueFile.com. The site hosts free high resolution digital stock photographs free for either corporate or public use.
As of November 2015, search "cash" using the search box on morgueFile.com to find several relevant results. Click on the thumbnail to bring up a pop-up window with a download link. To crop the image to suit your needs, click on the crop icon below the "Download Image" button.
Another option is to search Flickr.com photos taken by users that have chosen to offer their work under a Creative Commons license, which is used when a photographer wants to give people the right to share, use and build upon a photo they have taken. Flickr offers a separate search box for Creative Commons content. Click on the thumbnail to see the image in a higher resolution, and reveal a download arrow icon at the bottom right of the image.
A similar site to search for free images of stacks of money is Wikimedia Commons, a database of millions of freely usable images. Search using the search box on the home page to display a list of results. Like morgueFile and Flickr, simply click on the thumbnail to find a download link.