One interesting use of MicroStock is add images to older web pages that are central to your site or your site's message.
For example, the Community Color blog index is important to my work building community directories. So, today I spent a Microstock credit and slapped an image to the page. All of the different programs that crawl the web will note the addition of content.
After adding the image, I used the LinksAlive Microstock crediting program. The image on the page links to a page with the graphic design credit and back to the index.
With this trick of re-inforcing content, I am now tempted to go back through my blog and add relevant stock images to pages of import. Assuming, I can find some.