First of all, it’s completely false that any image you find by googling is free for the taking. You should always click through to the image and determine whether or not the source page owns the image and all its rights, or whether they’ve bought, borrowed, or stolen it from someone else.
If you find an image in someone else’s blog post, you should leave it alone unless they have verbiage that spells out attribution requirements, and/or provides creative commons info about the image. There are some exceptions, such as fair use of movie stills.
Your safest bet is to go to a site that sells or aggregates images, with their copyright info spelled out.
For free photos, these are my favorite sources:
MorgueFile – Free of charge, and copyright-free, but there appears to be no curation, so the imagery is of hit-or-miss quality.
PhotoPin – Free of charge, but the search filter is iffy, and the images aren’t always copyright free; you have to click through to Flickr to find out whether they’re “creative commons” — which means you can use them within certain parameters – or “all rights reserved” — in which case you can’t use the image without explicit permission from the owner.
For paid images on a tight budget, these are my favorite sources:
StockFresh – Prices start as low as $1, but are usually good quality and the site has good search function.
www.istockphoto.com – Huge selection and good quality; excellent search function; most images $36 and up. (They used to be a lot cheaper before Getty bought them out, but this is still inexpensive compared to the old days of rights-managed photo buying.)
Disclosure: Clicking on StockFresh or iStock links above earns me a bit, but I honestly use and like both these sources.