I found professional image libraries sites like http://www.istockphoto.com or http://www.dreamstime.com are quite useful to add some cool images to a website either when filling its contents or when designing its graphic layout.

Unfortunately both of the site I listed above use credits plans that expires after 12 months: you buy credits (using real bucks) and then you can download images, but if you don't use all the credits within 1 year, thay suck them out from your virtual wallet (I think it's really unfair, but too bad for you, that's their policy).

Do you know about other good image libraries sites (from your real life experience) that use credits to download images, but thay don't expire after 12 months?

Obviously I won't ignore your suggestions about any other image libraries sites.