Ran in to this issue after building the new site. I was attempting to upload a new Header Image via the built-in tools and kept getting an HTTP ERROR when doing the upload. The files would show up but with no image / error with the file. Did some Googling and found multiple people with this issue, including here. One fix that was mentioned, adding a line to your .htaccess file, did not fix the problem for me. Another, using the “Default to GD” WordPress plugin did work. Download the “Default to GD” .zip file here for upload to your WP site. Use this plugin at your own risk.