We recommend that you follow these best practices for naming your image files prior to uploading them into the Nexternal Order Management System (OMS).
1. Never use spaces or special characters in image file names, always use either underscores or dashes between words. Only letters, numbers, dashes and underscores should be used. If you use spaces, your customers may not see the images, depending upon what operating systems, browsers, and versions of each that they are using. Even if you see them and don't see a problem, they might not. If you use special characters you may not be able to upload your images into the system.
2. For product images, we recommend that you name your images something related to the product – perhaps the product name – and not some random date or another alphanumeric name. This will make your life much easier when managing the image files relating to your products, and it will facilitate search engine optimization. Further, a recognizable name tells search engines what the image is about, which makes it more likely that you'll show up in image searches.
3. Image file names can have up to 50 characters (including the suffix).
4. For merchants utilizing the legacy tools for uploading product images, If you are creating several different images sizes of the same image, for example for the image gallery, consider naming them the exact same thing (name of the product), and add _th (for thumbnail), _mn (for main), _lg (for large), and _mini (for mini-thumbs), so you can tell what product and size the image is, just by looking at the file name. Again this will make your life much easier when managing the image files relating to your products. (For merchants using the newer image management tool, which we highly recommend, this becomes irrelevant as you will need to upload only one image and the tool will do the resizing for you.)
For recommended image sizes for your store, please refer to this article.