In the past, I've always gone to the old search standby, google, when looking for images. It's always worked sufficiently.
However, I recently started using bing. It's the revamped search engine from Microsoft and it's wonderful for finding suitable pictures and other images for your blog, sales pages, etc. (always make sure you follow the copyright rules). The cool thing about a bing image search is that seemingly endless relevant search results appear. There is no endless paging through 10 results at a time. You just scroll down the page until you find what you're looking for.
The results are far more relevant and it's so much easier and quicker to find exactly what you're looking for.
I love bing image search!