So i had the same problem and i just fixed it today. You need to go to your FTP file transfer program, connect with server and then go the your upload folder. This is shown in the wp-content folder. Then right click on the folder and choose permissions. Or something like that i dont know what it is called in english ?? then change the number into 770 and click save. This allows WordPress to rewrite the data in that map. In other words it allows wordpress to upload images in that folder.