I have just changed mine Chris.
CP -- Settings and options on the left --- Option 2 choose file - on the left select where the file is, mine I put on the desktop easy to find,
Click on photo and save change.
I found all that quite easily Jan, but the choose file 'button' didn't work when I clicked on it. I've now tried it on my iPad and it works there. I now need to put the photo I want on my iPad.