In addition to the product watermarks that can be laid over product images, Magento allows you to customize the default image placeholder image, which is used when a product has no product image available to be displayed.
To see the default Magento image placeholder, you can create a product and simply not assign it an image, which will result in something similar to the result in the following screenshot:
You can add products to your website by navigating to Catalog | Manage Products in your Magento store's administration panel.
Note
As most e-commerce store owners will testify, it's best to include product imagery on product pages, but there may be occasions where you might like to sell products through your store and may not have an image immediately available, so this is a good way to reinforce your store's brand!
To customize your store's product placeholder images, navigate to System | Configuration in your Magento store's administration...