Have I to regster again when I participated last year ?
No, just use your login from last year. If your last participation was in 2016, you have to register again.
How can I upload my pictures ?
When you are logged in, you can upload the pictures by drag and drop.
Can I change the language in the contest system ?
Yes on the top right beside of the signup/login button or your login information, you find a small globe. You can chose between English, German and French.