Question on using product images for online marketing

Hi Everyone,

I'm planning to put up an online business and I need some advise regarding the product images that I will use.

1. Can I get images from other sources (e.g google images, other websites, etc..) and use that in my   advertisements? Will that have any legal consequence on my side?
2. If I use images that doesn't belong to me but I cited the source, will that be ok?
3. I believe laws governing plagiarism varies from country to country, what should be applied if I got an image let's say from a photographer/website residing in the US?

Thank you and happy new year.

"I believe laws governing plagiarism varies from country to country"
Well. Different countries HUNT illegal copyists different much...  

It ISN'T allowed to copy any images or music, without permision from the owner, with exception of VERY old things. I believe elder than 100 years  (70 ?).

But there are some websites which offer FREE material.
LOCK UP! Some material is offered free for NOT comersial use, but yours is comersial.

You say you will sell things. Some SUPPLIERS offer images free/cheap.
So ASK the owners.
Some brands LIKE their images to be shown, some even PAY the websites to show images