image copyright

i've seen some sites share some small images. nothing like a logo or anything.

a friend of mine told me that if you see an image on a site, as long as you modify it somehow then it is ok to use.

im trying to make a site, and i did a google search for some pics that i want to collage to use for the banner--nothing special.

is it legal to use these pics?

No. The image is still intelectual property of the site.

I think there's a law that says something like if it's at least 30% different, then it's not considered the same image. But that's 30% CHANGE to the original, so adding a background to it wouldn't count.

that seems kinda flimys. what constitutes 30%?
30% of the pixels being different are different colors?
so if i flip it horizontally then most likely i will get a 30% difference

There's another way to look at it. If you're not going to make a hugely popular money making site out of images you found on the internet, then I doubt the owner will come after you. Legal? No. But Of course, you could always email the owner and ask permission.