You can add cover images directly into your NetGalley account (either .jpg or .png files) when you are creating a title and adding in your content files.
Click "Choose Files" to browse your computer for the files to upload. Please note that cover images must be 8MB or less. For more information on accepted cover sizes, please see this article.
If you would like to replace a file at any time, navigate to your Title Details page and look for "Title Information" in the side-bar to the left of the Title Details page, and click "Upload/Preview Files." You can learn more about updating cover images here.
If you replace your cover image, your cover vote count will reset.