When I find a photo somewhere on the internet, without any explicit information of whether I can re-publish it on my own website, without any hint of who is the owner/author of that photo, can I still do it? I'm puzzled here cause I've seen like millions of websites, often very big, that repost photos, most probably found via google and it's VERY unlikely they bothered to look for and contact the author of that photos. Is every one of that sites likely to be sued at any moment? What about the case of forums and content provided by users - there is virtually no way of prevention here.
Legally, you can't republish photos without permission. However, millions of photos are licensed by their owners under Creative Commons licenses that do permit some kinds of reuse, and you can find them by using the advanced search options on Flickr or on Google Image Search.
Where photos are uploaded by a website's users, the DMCA should protect the website itself from being sued, if it's in the US and if it responds appropriately to DMCA takedown notices. The user can still be sued, of course.
At least some major newspapers regularly infringe copyright. What other people do isn't an indicator of what's legal but what they've managed to get away with doing.
Also, most forum posts hotlink to images rather than upload them to the forum's server. The question of whether that can infringe on copyright is an interesting one, because if an image is hosted by Alan, Beth hotlinks to it on Charlie's forum, and Dorothy views it on the forum, the only people who've copied the image (under a strict interpretation) would be Alan and Dorothy. OTOH convincing the judge that it's the correct interpretation might require an expensive lawyer.
I can only answer this from the US perspective. There are many photographs as well as artwork that falls into the public domain category. Additionally creative commons is acceptable if the publisher specifically says it okay.
You may not simply use the link to the pic as your image source, thinking that you are doing them a favor because this is a different infraction, bandwidth theft.
My opinion, not a legal ruling is if you have found the photo via google, you should make your best effort to track down the website or website(s) where the photo was published, which is easy on google. If you are planning to use the photo (make sure that there is no copyright posted at the website, or within the caption with an attribution.)- Google always seems to hedge saying that the image may be subject to copyright.
I will not defend you if you get sued, yet I would say that if you acknowledge the source of the photograph.., credit the creator, and provide a link back to the source where you found it and, you will generally be on fairly solid ground. I think people who specifically warn you that you have to be "so much more careful" than what everyone else is doing, may be in the stock photo trade.
I have read something that a simple link back to Flikr is not okay under creative commons you should cite the creator of the image and provide a link back specifically to that persons account (or specific image on their own account) on Flikr.
Yes, you should always seek the original work's author/artist/creator and get written permission.
In addition to using Creative Commons licensed works on Flickr, you may also search http://www.sxc.hu/ which is a free stock photography site. (Requires free login.)