To esmi, about your response

Thanks for responding to my problem! However, I’m not an expert, and even this was over my head:

Try changing the permissions on the wp-content directory to 766. If you still have problems, try 767, 775 or 777. Once your uploads are working, change the permissions on wp-content back to 755 again and check that everything still works OK.
http://codex.wordpress.org/Changing_File_Permissions

Where is the wp-content directory located? If you could guide me step-by-step that would really help.