One of the easiest ways I have found is to download the eSteem app on a smartphone and then you can change your profile and cover photos there from your phone's photo library. But the cover photo is a bit tricky and I recommend to do that on a computer.

Here is a short article showing how to do it directly on a computer: https://steemit.com/steemit/@allasyummyfood/14-steemit-lesson-how-to-upload-a-profile-picture-and-cover-photo

And WELCOME to Steemit! I hope you have lots of fun here. :)