To upload your Cover Art image, go to the drop down menu located to the upper right of your screen and select the Settings option. You'll land on the profile settings page. You can upload your cover art there. The cover art must be a square image with dimensions of 1400 x 1400. Be sure to click the Save Settings button at the bottom of the page to save your changes.
To upload your Header image, from the Settings area, click over to the Show & Podcast option located in the left side settings menu. From there, scroll down the page to locate the Page Appearance section. You can upload your header image there. The header image must be a rectangle image with dimensions of 1600 x 400. Be sure to click the Save Settings button at the bottom of the page to save your changes.
For tips and examples for creating great cover art and header images, please visit the following link: