Searching for images to use, particularly to liven up presentations and blog posts, can be a turn-off because of copyright restrictions.
Of course, there are ways to find photos that can be used for free. My personal favourite is to look though Creative Commons-licensed photos at photo sharing site, Flickr. They have a Creative Commons page which explains the different licenses. You can also restrict Flickr's advanced search to only include Creative Commons-licensed photos (scroll to the bottom of the search page to see this option).
Searching for images to use for free on the internet only begins from there. If you want to know more, there is a very thorough online tutorial called Internet for Image Searching.
The tour section of the tutorial has a useful list of sites where you can get images. However, I suggest that you do the entire tutorial so that you get a complete picture about the issues involved in finding and using images on the internet.