Here ya go Using pictures from the web: are there any rules?

There are rules. Essentially, any original content you publish to your site is yours, and likewise, any original content user X posts to his/her page belongs to him/her.

You want to reach out to a website owner and ask permission before using any images you are borrowing from a website, and see if it's ok. Most people are ok with it, so long as you give proper credit (usually a short "Photo by: X" and a link to the original)

But like createpeak mentioned, there are stock photo websites out there, some paid, some free, but the paid ones are going to provide you with a bit more quality and exclusivity (if its free, more people have probably used it, lowering the impact that image may have on your viewers)

I have even heard of users seeking out photographers via a paid stock photo site and asking for commissioned work. You may pay a bit more for this, but you can negotiate the terms of use and with the right price, ensure you are the only one who can use those photos, aside from the photographer using the photos in a portfolio.

There are rules, although, it may or may not be reinforced so much, depending on how popular your website is. Quality stock images are expensive. If you use the ones on a Google image search, you could be taking a chance. I'm sure, however, there are many bloggers who do this every day.

Try this source: http://budgetstockphoto.com/

Not sure about images linked to other sites but you can Google almost any images and find sites that provide free images.