In the Contributor Portal, select the Portfolio tab at the top and then click on any of your files. That allows you to choose the option "Make profile cover image" for your selected file. Your cover image will be replaced by the new one. There's no option to go back to the grey background.
Thanks Kirsten but it's necessary to improve these functions.
The resolution of the profile cover is not good at all. I'd rather be able to have it removed then it to be like this, it looks amateurish.