Is it fair use if I give credit for a photo I use on my blog?
No. The U.S. Copyright office states, "Acknowledging the source of the copyrighted material does not substitute for obtaining permission."
In almost every case, using a photo in its entirety, in conjunction with a blog post, is a violation of the rights of the copyright holder. The one fairly clear exception to this is if the blog post is a critique or analysis of the photo. The rest of fair use rules are a murky mess that typically sides with the rights holder.
To avoid the possibility of violating copyright when using images your best bet is to use images with clearly defined rights. Getting an account at iStockphoto.com is a great way to do this. The x small sized photos at iStockphoto are typically large enough to accompany most blog posts and are licensed royalty free as part of the contract with iStockphoto. At $1 per image, the x small iStockphoto images are also affordable. Flickr includes over 17 million photos licensed under Creative Commons Attribution License, which means the copyright holder has explicitly given permission to use the photos as long as attribution is given.
There are literally hundreds of other royalty free photo sites and online sources of Creative Commons licensed images, but Flickr and iStockphoto are probably the two that are most accessible to everyone from both a cost and ease of use perspective.