Put this in a .htaccess file in the uploads <dir>:
AuthUserFile .htpasswd
AuthName "Upload Directory"
AuthType Basic
<Limit GET POST>
require valid-user
</Limit>
snd put this in a .htpasswd file:
#replace <username> and <password> with actual details, also you can encrypt the password with a free online tool. Also I reccomend you find a way to restrict filnames with '.ht' in them.
<username>:<password>