Yep. It's not a size issue. No server errors, either.If it was an image size issue, it should tell you the image is too large for the server to process.
Does the console give you any information? (firebug or Chrome developer)
Are you able to upload the same image here?
Yes it is, but the rest of the Xenforo install works fine.another thing....is the sandbox site behind .htaccess ?