You can upload a cover image directly under "edit profile". There you will find the button "upload cover image" and you can also adjust and then save your image.
Only cover images in JPG format and with an image size between 50KB and 5MB are allowed. The resolution may not exceed 50 megapixels (that would be a maximum image size of 5000 x 10000 pixels).
We always check photos before accepting them according to the same criteria as profile photos.