If you would like to reference non-product images from your Merchant sub-directory, you can upload them to the graphics folder by using the "Body Background" image location.  You can use any other image upload box you like, but
the "Body Background" box is easy and if you forget to click "Reset" you will definitely know it :o)   (Remember you should always check your store anytime you make changes.)

Login to admin, and click on the hyperlink for your store, then click on the "Images tab.  The "Body Background" image box is the first one on the page. 

Use the "Upload" button to locate your image and when you are done, simply click "Reset" at the bottom of the page.  You can even upload many images in a row and click the "Reset" button just once at the end. 

To reference the image you just uploaded... copy the path that is generated in the in the image path field BEFORE pressing the reset button.  You can then paste this string anywhere within the merchant storefront and your image will be shown.  

Remember to press the "Reset" button, or empty the box of characters if you don't have another image that you are actually using for your site background.  Otherwise your site will be updated with the 'background' that you last entered into the box!