Facebook, images and copyright?

[Splintercell]

Greetings,

I'm am a little confused when it comes to the rules and laws of copyright infringement when it comes to social media. One can simply find a picture on google search, save and share it..or they can just repost images and share directly from Facebook. You can save images from pintrest and post and share it on your Facebook wall. But aren't these violating copyright laws? If I saw a nice pic of kittens prancing around and I shared it on facebook and told my customers "It's Friday!", isnt that copyright infringement?

I have a business, and apart from creating my own content, I find it difficult to know if i am allowed to share existing pictures that I generally find on the internet, because I never know if i infringe or not.

Are there any rules that I can follow?
Do you know the rules?
Is there a website where I can access images from and share it without any worries?

Thanks