To add or change your cover image, hover your mouse over your profile picture in the top right corner of the screen when you're logged in, then click on your name.
From there, you can click on the 3 dots just below the lower right corner of your cover photo, and there you can select if you want to Upload a Cover Photo.
You'll then be able to upload an image from your device.
A good aspect ratio for the cover image is 2000 px x 300 px or 1500px by 300px depending on your cover photo details.
For help or questions, please click on the purple chat icon on the lower right side of your screen, then "Customer Support" or contact help@legiit.com