Unable to change theme?

[Horsenround]

I am unable to change my theme at all. If I select anything different, I can't upload a new hero image for the top of the page, and it doesn't appear to be requiring a different size anyway. The sample thumbnails for each of the themes have different sizes for the hero image. I have created several images the size called for (2880 x 1144) and smaller, but it errors out without uploading. Ideas? Is anyone else having this trouble?