Durk_Tank you might not gonna find it...so let me explain it to you but before that I only uploaded the cover by using my phone...
I hope it will help you...here are the steps:
1: open your inkstone link in your writing section (info).
2: after the page open when you log in take alook on your left and you will see the 3 icon above. Press the $ icon ( the #3).
3: when the page pop up, press the middle column icon, the stories toggle would appear. Press the stories toggle.
4: it would show your writings...novel..
5: on your right side, you will see the explore toggle, press it.
6: you will see the setting icon up right and press it.
7: all the details about your novel will be showed up.
8: the upload toggle is just below the cover picture and you know what to do about it.
9: follow the requirements and upload your cover.
10: please save your alteration down to the Webnovel icon. Within a few second you will see the change..
Good luck!