Compfight is a home brewed flickr search tool specifically for the advertising / design community. The rails based ajax search bar pulls in a flickr feed at speed so that you can quickly scour the site.
You can restrict your search to the Creative Commons with “only”, and use the “commercial” setting for commercially usable assets. Blue bars indicate flickr is holding an original. Linger your cursor over the image to display dimensions. For nothing but originals switch Seek Original to “only.”