Yes, even I can do it. Look across to the bottom right of the web user interface, next to the selected album title. You should see either a small pic of the CD cover or a blank square. Click on it and look across to the bottom left for instructions. You can add your own saved pictures but they have to be jpg files I think. You can replace a picture which was loaded as part of the ripping as well as simply filling any gaps.
Alternatively, if you were to rip your CDs on a computer using dBpoweramp software, or similar, you could edit/add art more easily before the rip.
Finally, there's an option in the UI to click on the three dots next to any album title ( top right) and it will offer a couple of other options for finding art.