That said, you probably wonder why you find so many photos on other blogs that don't cite any sources and have apparently been copied from somewhere off the web. Well, many just don't know that this is asking for trouble and others simply don't care, figuring that chances are infinitesimal small of anyone finding out. Still, this obviously doesn't make things right!
Respect the artist's work and don't steal it! Of course it doesn't hurt to ask, and many photographers or illustrators will even allow you to use their work as long as you mention who created it and/or link back to their own website.
Familiarize yourself with a website's FAQs! There are sites who will allow you to share some of their content, be it text and/or photos. Comply with their rules, eg linking back to the original post, and you're all set.
Protect your own work! I've seen so many photos on blogs that show no watermark or anything that would make it possible to track its source. Stealing those is way to easy, so put your stamp on it. Even if you have nothing against other people sharing your images, I'm sure you wouldn't mind that people who find it elsewhere can at least track its source.
Stick to Stock Photography! If you aren't handy with a camera or if you need a photo you couldn't make yourself for one reason or another, use stock photos instead. Apart from sites that will charge you, there are also some great ones completely free of charge. One of my favorites (also used above) is FreeDigitalPhotos which gives you access to thousands of photos and illustrations.