Lots of people will know these websites but these seem to be some of the best and most commonly used websites.
Google Image Search
I really like googles new filtering tab on the left as this makes it much easier to taylor your search. The colour filter is particularly clever.
Flickr is by far one of the best if not the best image sharing website on the web today. Flickr has got millions of photos and the best thing is that some are creative commons share as you like meaning you dont have to have special permission to use them. The search could be a bit better I think though.
This is one that i just found recently. I really love the user interface and the way that it makes it easy for you to view the images. It is a stock photo website just to let you know.
There are loads of other great sites and you can have a look at this post here that has even more links to cool photo search engine sites.
15 Best Image Search Engines to Find Photos Online
The last thing i want to leave you with is a cool online photo editing website that I found.
This is a great site that is free and easy to use. You dont neet to even create an account it just works. You can edit photos and make them look really cool and funky or just touch them up to improve the way they look