Updating the Blog

I am having a tough time with this blog. I am trying to update the picture at the very top. While it is an adorable picture of Isaac, I'd like it to show both kids. Does anyone know how I can change it? When I tried to add a new photo, I got this:

I thought maybe I should just buy some storage space, but when I went to do that, I saw this:

I think I should have PLENTY of room for a new photo, don't you? :-) If anyone knows how to fix this, please tell me. I've tried to update it a few times without success. There's no way to contact Google to get help with it. I have "Sent Feedback," but it basically says that I shouldn't expect a response. Thanks for any suggestions!!!