Heya,
Here are the pages on the main site to check out:
https://www.lddb.com/help/submit/ for the submission format and restrictions, and
https://www.lddb.com/help/cover/ for a guide on image editing for quality assurance. The guide is for IrfanView; I myself use GIMP when I'm submitting covers, but the guide is great if you're not already accustomed to an image editor.