Sometimes Weebly has difficulties loading the background image if there are a lot of existing elements on the page. What you can try to do is create a new page, and upload your background image that way. This should work without any issues. Once that’s done, then copy the elements from your existing page into the new page.
If you have a BLOG, here is a helpful tip contributed by Stefani Alexander!! Thanks Stefani!
I was able to get a new background to load by changing the settings on my blog to only have one post per page. It allowed me to have less on the page without deleted my blog. The image finally loaded.