The copyright low to respect is where the content/material is copyrighted, so Argentinian lows doens't metter. I can only say that use free images is better and no one can claim anything.
I never seen copyright owner ask for money or claim anything, but if you're taking images from famous portals (nulled) you can have a lot of problems, since this sites has an authority.
This site it's great and free: http://pixabay.com/
Ah, giving them credit and possibly a link on your website is a good idea! That could probably keep some of the smaller copyright owners at bay, but the larger companies will not humour it.you can add a link where if one feels there image are copied they can apply there and you can take the image down... or give them credits on your website to avoid any copyright issue