Hi Spartan91,
I can't speak for Oscar, but I can offer some help.
Google's use of online images in its "Image Search" falls under "fair use" technically. That is because Google doesn't serve up the images to you directly; it simply links to the location of the actual images. There are those who challenge this thinking, because Google must temporarily store or "cache" the images on its systems at least temporarily in order to provide you with such a search function that includes those little thumbnail pictures (but this is beyond the scope of this post). However, that doesn't give anyone the right to use an image just because Google lists it in a search. In fact, it doesn't matter where an image came from... a web site, a Google search, or even scanned from a book... if you use it without license, you might be held liable, or at the very least, harassed for a long time (you'd think that you've stolen a Ferrari, for God's sake). Furthermore, if a person or organization uses an image without license and then someone else gets it from them, that wouldn't offer the latter any indemnity. There's also no provision in most places that state that an image must be protected by watermarks, copyright warnings or metadata in order for it to be protected by copyright. Of course, for an entity to sue you and actually receive payment, it should have properly registered the images for copyright beforehand. Unfortunately, it's often difficult to determine what's formally copyrighted and what isn't on the web. Technically, everything that anyone creates is copyright "the author", but it's difficult to get damages in court with that, unless somebody used an unauthorized picture of your likeness in an advertisement or something... you could sue and get damages in that case I'm sure.
I'm not a big fan of the stock image/art companies. However the above would be considered standard in many circles these days. To play it completely safe, you should make images yourself (if you are so talented), pay someone to do it for you, or buy images from a company that you feel offers good value and conducts itself in the most ethical manner.
In the case of separate companies selling identical images, I would guess that it would come down to which one has actually applied for and received copyright on the particular image. That would be the company that would "own" the image and could best protect its use legally. The other companies would be dead in the water in such a case. It's also my opinion that if none of the companies have formally copyrighted the image in question, then none of the companies would be a a strong legal position to protect it.
S.