How to change the product image placeholders in Magento
When you add products to your catalog in Magento and you don't upload an image(s) for that product, a default no image picture, also referred to as an image placeholder, is displayed on the frontend of the site. By default, this image is a light gray background with the Magento logo. You can change that image placeholder with any image of your choice.
One way to do it is through the admin panel of your Magento. After you log in, go to System>Configuration, on the page that opens click on Catalog under the Catalog menu on the left, and from the panels that appear on the right click on the Product Image Placeholders panel to expand it and see the options. Use the respective Browse buttons to locate the image on your computer and upload a Base Image (shown on the product details page), a Small Image (shown on the product list page), and a Thumbnail image (shown in product thumbnail galleries and in some blocks). Then click on the Save Config button in the upper right corner. The images will be saved in public_html/media/catalog/product/placeholder/default on your hosting account (assuming that the root Magento directory is public_html). These images will apply to any theme. It doesn't matter what the dimensions of the original images are; Magento will automatically resize them before displaying them on the frontend.
You can also change the default image placeholders manually by uploading the desired images to the appropriate locations on your Magento hosting account. Assuming that your Magento is installed directly in the public_html directory on your hosting account and, for example, you want to change the image placeholders of the default theme that comes prepackaged with Magento, you should go to public_html/skin/frontend/default/default/images/catalog/product/placeholder. There you'll find three images: image.jpg, small_image.jpg and thumbnail.jpg. Delete them, upload the images that you want and rename them with the same names as the original ones. You can do that from the Files section of the Pixie control panel or with an FTP client (e.g. FileZilla). Note that this will change the image placeholders only for the particular theme (e.g. default). If, for example, you want to change the image placeholders of the theme modern, you should change the images in public_html/skin/frontend/default/modern/images/catalog/product/placeholder.