Cover images should be high resolution, and 8MB maximum. At the minimum, the longest side should be no smaller than 500 pixels. We only accept .jpg and .png files. Whichever file type and whatever size, please make sure that the files are optimized for the web.
NetGalley automatically resizes the cover images to fit within our standards, while retaining the same aspect ratio for use throughout the site (for example, as thumbnail images). Our recommended aspect ratio for images is 5:8.
If your cover image is not appearing true to color, double check that the color space is RGB, found in the file metadata. CMYK is generally used for images that will be printed, and may result in your cover image appearing off color when uploaded to NetGalley.