You need to change the folder permissions (or CHMOD) for the ‘upload’ folder in the ‘wp-content’ folder. Best would be to just change the whole ‘wp-content’ folder since you need the changed permission for plugins and themes as well. If you use a FTP program like Filezilla you can do it like this: http://www.flashimagegallery.com/images/chmod_settings.gif
In other programs there should be a similar setting.
Hope this helps 🙂